Implicit differentiation.

There's a multi-step problem that I need help in figuring out how to do.

"a) Find the equation of the tangent lines to the circle $\displaystyle x^2+y^2=25$ at the points where $\displaystyle x=4$.

b) Find the equations of the normal lines to this circle at the same points. (The normal line is perpendicular to the tangent line at this point.)

c) At what point do the two normal lines intersect?"

Could someone tell me how to start the solution to these questions, please; i.e., what do do? Thank you.