The speed of a 5kg cart changes from 12m/s to 34m/s in just 5 seconds.
What is the force acting on the cart?

Step 1 Calculate the acceleration.
acceleration = change in speed/period of time
Change in speed = final speed - initial speed = 34 -12=22m/s
Period of time = 5 seconds
Step 2 Acceleration = 22m/s/5/s = 4.4m/s/s

The force = 5kg X 4.4m/s/s = 22Newtons

Incorrect

The formula you should use is
F =ma

The acceleration is the change in speed over time.
Calculate the acceleration.
(Initial speed - final speed ) / time taken (seconds)=acc.
The mass of the cart is given as 5kg.