2 body 1 dimension problem

[abcba]

Hello, I'm having a bit of trouble starting this problem:
A ball is dropped from a height of 30 m. At the same instant another ball is thrown straight up from the ground with aspeed of 30 m/s. How long are the balls in the air before they reach the same height?

I'm not sure what equation to use here. Any help would be appreciated. Thanks in advance.

[HallsofIvy]

You should have the equations governing the height of each ball.

The first ball has initial height 30 m and initial speed 0 m/s (it is "dropped"). Its height at any time t (in seconds) is h(t)= 30- (g/2)t2 (g= 9.8 m/s2).

The second ball has initial height 0 and initial speed 30. Its height at any time t is
h(t)= 30t- (g/2)t2. "they reach the same height" when h(t)= h(t) for the two formulas: 30- (g/2)t2 = 30t- (g/2)t2. Solve that equation (which is remarkably easy) for t.