I'm stuck on this question:

Find the equations of all lines tangent to y=x^2-4 that pass through the point (5,5).

I found the derivative of the function (y'=2x), and plugged in 5 to get a slope of 10. Then I put it into the y=mx+b form to solve for "b". I got y=10x-45. But when I graph the original function and the equation of the tangent line, the tangent line isn't... tangent. Help, please!