Imagine a leaf floating down a creek (south) at a rate of 6 m/s. you walk in the creek towards the leaf (north) at a rate of 2 m/s. relative to the ground, at what rate is the leaf moving? down arrow 6 m/s south + up arrow 2 m/s north = ?

Because you are moving in the opposite direction to that of the leaf, we subtract your velocity from that of the leaf and that gives the rate at which the leaf moves relative to the ground.

This implies that

Hence it the leaf is moving at

the solution is 12x-6y