Explain why action-reaction forces do not cancel each out

I "love" these sorts of questions ... 
Book on a table. Book presses down on table as the action. Table reacts by pressing up on book. So, the forces are on different objects.

Object colliding with, a very heavy obstacle such as a brick wall. Object presses on brick wall. Action. Wall presses back on object. Action- Reaction equal and opposite and on different things. So, object gets a "recoil impulse of force" from wall and bounces off".Wall gets a "recoil impulse of force" and hardly moves 'cos of its much bigger mass (which is why I chose it for the example).

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.

Distance^2= yellow line ^2 (see picture) = 30^2 + 49^2so distance = squareroot of (300^2 +49^2)i dont know if this is how to solve it, but i cnt think of another way