I would assign some x-values if I were you. Suppose is the point at which you're taking the tangent and normal lines (which we will eventually need to be able to move, so just think of it as "temporarily constant"), let be the x-intercept of the tangent line, and let be the x-intercept of the normal line. Because you have assumed and you know that Why?
Then you want to set
Your goal is to eliminate and How can you do that?