The first question, which you know how to do, was substituting a known value for L and solving for V(L). In the second case, you are given V(L) and asked to solve for L, so you just have to rearrange the equation so that L is on one side by itself, and an expression involving V is on the other. Then substitute in the given value for V in the second part, and solve for what L would be in that case.
You can say that you are "inverting" the formula, to turn it from a formula for V as a function of L, into a formula for L as a function of V.
The first step in that process is to get L out from under the square root; in other words, you have to "undo" the square root, by doing the inverse operation. What is the inverse operation of taking the square root (if you have the square root of x, how do you change that into x?)? You have to do that to both sides of the V(L) expression first, then rearrange to get L on one side by itself.
