help ! how to calculate the horizontal and vertical components of the acceleration of a truck?

The x acceleration should be negative too, since it starts with a high velocity in that direction and ends with 0.

The acceleration on the 27 degree slope is given by

v = u + at where v = 0, u = 27.8 m/s, a = ? and t = 5.8s

0 = 27.8 +a(5.8)

a = -4.8 m/s/s

Dividing this into components, the x component is a cos (theta) = -4.8 cos 27 = -4.3 m/s/s

The y component is a sin (theta) = -4.8 sin 27 = -2.2 m/s/s

Hope that's helpful.

Draw a reference triangle. The angle of elevation is 27. The hypotenuse length (total acceleration) is 100/5.8. From there use sine and cosine to find values, the assign the appropriate sign to each (negative).

Simply use trigonometry and the formula for acceleration.  Of course gravity will be split into x and y components because of the given angle.  So use tan27degrees and solve for the y component of gravity acting on the truck, and then use cos27 to solve the X component of the trucks acceleration    Then calculate the acceleration of the truck will be calculated.  Acceleration is equal to final velocity minus initial velocity divided by time.  Here we go

Acceleration of truck is equal to -17.24 meters per second squared  note this is the hypotenuse component of the truck's acceleration.  X and Y still need to be solved.
Acceleration of gravity in the y component is still the same, 9.81 m/s squared.

Acceleration of gravity in the y component is 9.81m/s squared (down)
Acceleration of the truck in the y component is approximately 7.827m/s squared (down)

Acceleration of gravity in the X component is -19.25m/s squared.
Acceleration of truck in the X component is -15.36m/s squared.

Now we simply add our like terms,(x and y components).  

So total force in the X component is -34.61m/s squared
And total force in the Y component is 17.637 m/s squared (down)

Tips:  Always assign a direction ( up and down as positive or negative)
Make triangles for each force of acceleration.  It will make these types problems a breeze =)

Hopefully this answer is right, be sure to let me know, good luck.