For solving this we need the change in distance and the change in time. You said that the change in time is 1.8 seconds and the change in distance is 3.6 meters. ( This is assuming it started at 0 meters and started at 0 seconds )
Since the starting was 0,0 we can toss them out as they add no value. Now since speed is labeled (in this case) m/s, we divide how many meters it traveled over how many seconds it took to travel it.
3.6 / 1.8 = 2
The average speed of the cart is 2 m/s