Answer
$t=[\sqrt {v^{2}_{0}+2gh}-v_{0}]/g$
Work Step by Step
We know the following equation: $v_{f}=v_{0}+gt$
We substitute the answer from part a: ($v_{f}=\sqrt {v^{2}_{0}+2gh}$):
$\sqrt {v^{2}_{0}+2gh}=v_{0}+gt$
We subtract $v_{0}$ from both sides: $\sqrt {v^{2}_{0}+2gh}-v_{0}=gt$
Finally, we divide both sides by $g$: $t=[\sqrt {v^{2}_{0}+2gh}-v_{0}]/g$
