The speed-time graph of a cart is shown on the left. A 4.5N force acts to slow the cart down from 23m/s to 10m/s.
What is the mass of the cart?

Step 1 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
Acceleration = -13m/s/14/s = 0.93 m/s/s
Step 2 m= f/a = 4.5/0.93 = 4.84kg

Incorrect
Some hints.
The formula you should use is
m=f/a

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.