Explain why an object moves if the action-reaction forces are equal.

An object moves even though the action-reaction forces are equal because not all objects have the same inertia, which is resistance to motion. This is why when you hit a ball with a bat the ball goes flying while the bat is only pushed back a bit. It is also why whenever you stand on earth, you're technically pushing the earth down a very small amount. You just can't notice it because the inertia of the earth is so great. 

The boat's velocity relative to the riverbank can be resolved into two components. the component going north is 2.7 m/s and the component going west is 1.2 m/s.we add these two vectors and then find the magnitude of the resultant to get the speed.the magnitude of the resultant vector from adding can be obtained by the pythagorean theorem.    to two significant figures, the boat's speed relative to the riverbank is 3.0 m/s.