How much must I accelerate driving 55 mph to get to a complete stop?

Trick question, huh? Well, there isn't any info to indicate the answer can be anything but: Not at all.
You could speed-up, slow-down or maintain 55mph and eventually hit the wall, regardless.
And now that I've written that I'm less than positive.So I await the verdict from wiser answerers.

first, lets get the numbers sane:

55 mph = 24.5872 meters / second

47.3 meters at 24.5872 meters/second gives us 1.92376521 seconds.

So if you continue at your current rate, you will hit the wall in 1.9ish seconds.

This means you must come to a complete stop in that amount of time.  You want to accelerate from 24.5 meters/second to 0 meters/second in a period of 1.9 seconds.

You're acceleration delta is 0-24.5 meters/second = -24.5 meters/second.

-24.5/1.9 = -12.8 meters/(second^2)

Therefore, you must accelerate at any rate less than -12.8 meters/(second^2) to avoid hitting the brick wall. (note this must be the average accelaration rate over a time of 1.9 seconds)

If it were my car, however, anything greater than -15 would be unacceptable.

55 Mph is  24.5872 m/s

v = at + 24.5872
you want v = 0 to stop, so 0 = at + 24.5872
a = -24.5872/t

position = (a/2)t^2 + 24.5872t
or, (a/2)t^2 + 24.5872t  = 47.3

plug in a from above gets you:
-12.2936t + 24.5872t = 47.3

12.2936t = 47.3
t = 3.84s

therefore, a = -24.5872/3.84 = -6.39 m/s^2, meaning DEcelerate at 6.39 m/s^2 (minimum)

Your acceleration must be at least -6.4 m/s² in order to avoid hitting the wall. Note that your current speed = 55 mi/h / 3600 (s/h) * 5280 ft/mi * 12 in/ft * .0254 m/in = 24.5872 m/s. Assuming constant acceleration, your average velocity over the deceleration period will be half that, or 12.2936 m/s. At that speed, it will take you 3.8475 seconds to hit the wall, thus you must go from 24.5872 to zero in that time to avoid hitting it. Thus your acceleration must be -24.5872 m/s/3.8475 s = -6.4 m/s²

Frankly, it's a badly worded trick question, so the answer could be anything..................... ..........................

But here's a proposed answer anyway:
If you are traveling at 55mph and a pile of bricks is 47.3 meters ahead of you, you can stop short of the bricks by accelerating at
-6.39 meters per second per second. (Note that the negative sign indicates DEceleration.)
This will bring you to a stop in 3.85 seconds while traveling 47.3 meters.

Incidentally gravitational acceleration is 9.8 meters per second per second, so we're talking about a deceleration force of about two-thirds of one g. Your tires and brakes are probably capable of providing this stopping force. And while your passengers will definitely notice that something is happening, they will not be hurt by that rate of deceleration ... provided they are belted in and don't become flying missiles within the vehicle.

Now, what was the trick answer that you had in mind?

hate to spoil things but acceleration can be both positive and negative.. so just accelerate in a negative manner,, to 0

No acceleration is necessary.  Stay at 55 miles per hour and you personally will probably hit the windsheild and the bricks and the road.  Nothing will be moving at that point!