Do you mean that your production function is or ?

I suspect it is the former. If you know derivatives, you can take the derivative and set it equal to zero. If not, you need to complete the square and the maximum will be at the vertex since this is a downward facing parabola.

Thus the vertex is at

So for part a, X is maximized when L=50

For part b solve

Alternatively you could have solved

which yields the same two answers for L.