What you must do is find at what point a line is tangent to the parabola and passes through (100,50).

You can use to find your parabola equation.

You need to find the derivative of y to get the slope m.

In doing so, we find the equation is

Now, you have a 'passes through' point but do not know where it is tangent.

You can use

Now, solve for x. Only one answer will be within reason.