To ward off oceanic invaders, a cannon is placed 10 meters from the water's edge, pointing out at sea. The cannon is capable of firing a projectile at a maximum velocity of 100m/s.

a) what is the maximum firing range of the cannon?
b)If the cannon was placed on a cliff that was 250m high, how much farther would the cannonball be able to travel?

Maximum range is achieved at a 45° angle of elevation. Just use your equation of motion to get the range. If you have forgotten the relevant formulas, read a nice discussion here.

https://en.wikipedia.org/wiki/Trajectory

Then add a term of 250 to adjust the equation.

Hmmm. Is maximum range still achieved at the 45° angle of fire if the destination is lower than the initial point? Maybe a shallower angle of travel translates into a slightly increased range.