[Coda]

You will always get two answers for a quadratic (that is, containing x²) equation unless the apex of the parabola is exactly on the x axis.

Do you already understand the physical significance if one value for t is negative? There's a meaningful answer but I don't want to do your homework for you.

If both values for t are in the future, there's a good reason you're confused: it PROBABLY means you've made a mistake somewhere. Either you're not considering the ground to be 0, or you've got the sign for one of the initial parameters wrong (the initial position, the initial velocity, or the acceleration), or you have an error in your computation somewhere, or you have one of the parameters wrong.

In this case, you've got the parameters wrong. Look at the formula more carefully:

x_f = x_i + v_it + (1/2)at²

I'll give you three hints:

1. If the height of the cliff is h, then what's the position of the boulder when you start looking at it?
2. What's the value of v_i?
3. You don't have enough information to solve this all with a single equation.