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.