The speed-time graph of a cart is shown on the left.
What is the mass of the cart if the force acting on it is 3.5 N?

Step 1 Calculate the acceleration.
acceleration = change in speed/period of time
Change in speed = final speed - initial speed =
40 -10=30m/s
Period of time = 14 seconds
Acceleration = 30m/s/14/s = 2.14 m/s/s

Step 2 Calculate the mass of the cart using the formula m= f/a => 3.5/2.14 = 1.64Kg

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.