Using Volume to find Depth of a Volume

I calculated a volume: V = Length * trapz(X,Y) by being given the value of X as well as the formula of Y(X).

Now I want to find X when given a value of V, where the volume can become a function of Y [V(Y)] that can be solved using the bisection method. How am I supposed to get V as a function of Y to be solved if I don't have an X that I can use in the integration?