There are four basic kinematics equations for motion under constant acceleration. You are going to use two of them here.

For the first part we know that

So set the zero level at the top of the volcano and upward as positive. Notice that at the top of the motion the speed is 0 m/s. So we get

which you can solve for .

For the second part we also know that

and again v = 0 m/s so

You know from the first part, so now just solve for t.

-Dan