How do i calculate the acceleration of the cart t=3.0s

Acceleration is the rate of change in velocity (speed in this context).

on a graph of speed versus time, the slope of the graph for a straight-line segment would represent the acceleration in that entire segment.

we can see that t = 3.0s is in some sort of sloped interval of decline.

that's a constant rate of decrease in speed. so acceleration is constant in that interval. since acceleration is the slope, we can find the slope to find the acceleration at t = 3.0s.

slope formula: we have points (3.5 s, 0.5 m/s) and (2 s, 1.6 m/s)

the slope of that interval is

   

-0.73 m/s²