A cannonball is shot into the air with a verticle velocity of 20m/s and a horizontal velocity of 12m/s. How long before the cannonball returns to earth?

The horizontal component of velocity doesn't affect at all how long the canon ball is in the air, so you can ignore that. You only need to work with the vertical velocity, and you can use:

y = y_0 + v_i t + \frac 1 2 a t^2

Here y = y_0 = 0, v_i = 20 m/s, and a = g = -9.8m/s^2. Solve for t.