If you were in a canoe on a calm lake and you threw a heavy stone as far is you could out the canoe what would the canoe do

This is conservation of momentum. At the beginning the total momentum is zero. When you throw the stone you give the stone a positive momentum, which means you must have a negative momentum (when I say negative I mean in the opposite direction of the direction of the stone) so that the total momentum is still zero. You and the canoe will move in the opposite direction of which you threw the stone.

d.7.5cm is your answer