The speed-time graph of a 3500g cart is shown on the left. A force acts to slow the cart down from 23m/s to 10m/s.
What is the force acting over the 14 second period?

Step 1 = Change mass into kilograms
3500g = 3.5Kg
Step 2 Calculate the deceleration.
deceleration = change in speed/period of time
Change in speed = final speed - initial speed =
10 -23= -13m/s (the negative sign indicates deceleration)
Period of time = 14 seconds
Step 3 Acceleration = -13m/s/14/s = 0.93 m/s/s
The force = 3.5kg X 0.93m/s/s = 3.25 Newtons

Incorrect
Some hints.
The formula you should use is
F =ma

The acceleration is the change in speed over time.
Calculate the acceleration.
The acceleration is the gradient (slope) of the speed-time graph.
(Initial speed - final speed ) / time taken (seconds)=acc.
The mass of the cart is given as 3500g.
Change the mass of the cart into kilograms.