What is the acceleration of a race car starting from rest accelerates to 28 meters per second in 7 seconds?

Acceleration and deceleration is the time rate of change of velocity.

So, subtract one velocity from the other and divide by the time.  Remember that deceleration will be a negative value.

your question uses a really basic equation of non-uniform motion

a= (v2-v1)/t
In your case v2 = 28 m/s, v1 = 0 m/s, and t = 7 s
so a = (28-0)/7=4 m/s^2.

In Physics there is no such concept as "deceleration", rather we use "negative acceleration". "Negative" because it is in the direction opposite to the direction of motion.

So the same formula applies: except that you have v1 = 30m/s, v2 = 0m/s and t = 6 s. Plug those values into the same equation and you get
a=(0-30)/6 = -5 m/s^2

so that truck accelerates at -5 m/s^2, and it is then "colloquially said" that it decelerates at 5 meters per second squared.

vf=vi+at
28=0+7a
a=4m/s^2

The acceleration formula you would use would be:

 final speed - beginning speed
                        _________________________
                                         time

Then you plug in your numbers:

28 m/sec - 0 m/sec
                        _________________
                                  7 sec

Do subtraction:

28 m/sec
                        ________
                             7 sec

Do division:

Acceleration = 4 m/sec^2


There is no such thing as deceleration. Deceleration is just accelerating the opposite way. If you were going north and slowing down, that means you are accelerating south.

The formula you would use would be:

final speed - beginning speed
                        ________________________
                                         time

Plug in your numbers

0 m/sec - 30 m/sec
                        ________________
                                   6 sec

Do subtraction:

-30 m/sec
                        _________
                              6 sec

Do division:

Acceleration = -5 m/sec^2

suppose the truck was going west. Then this could also be written as:

Acceleration = 5 m/sec^2 east

because it is accelerating east.

You've learned above how to calculate the answers to your ?.

Please feel ENTIRELY free to use the word deceleration.  It's a perfectly valid term listed in Webster's Dictionary as a term nomally objected to only by anal physics majors.  Everyone else understands it intuitively.