This is a pretty simple physics problem, but I do not know how to go about it.

How do I:
Show that the time required for a projectile to reach its highest point is equal to the time for it to return to its original height?

The maximum height is where V = 0.

The acceleration rate is -g, in either direction.
The initial speed is the same as the final speed (because of conservation of energy.)
The time it takes to decelerate from initial speed V to 0 is V/g.

The time it takes to accelerate from 0 to -V is also V/g

This works whether the projectile goes straight up or not. V represents the vertical velocity component.