Achild riding a bicycle at 15 m/s decelerate at the rate of 3.0 meters per second squared for 4.0 seconds. what is the child's speed at the end of the 4.0 seconds?

3m/ of deceleration for 4 seconds.
3 x 4 = 12
15 - 12 = 3
The child will be riding a bicycle at 3m/s after 4 seconds.

If you want a full equation (which you might not):
let a be acceleration rate, Vf be final velocity, Vi as initial velocity, and t as time.
a = (Vf - Vi)/t
let's fill in everything we know now:

-3 = (Vf - 15)/4
now let's solve for Vf

-3 = Vf/4 - 15/4
-Vf/4 = 3 - 15/4
Vf/4 = -3 + 15/4
Vf = -12 + 15
Vf = 3

Therefore, final velocity is 3m/