I can't figure out how to solve this problem:
finf k such that the line is tangent to the graph of the function.
x^2-kx y=4x=9
Can someone help me understand this problem?
Did you mean that we were to find k, such that $\displaystyle y = 4x + 9$ is a tangent to the curve $\displaystyle y = x^2 + kx$?
If so, remember that the derivative of a function gives the equation of the tangent of that function.
If you differentiate $\displaystyle y = x^2 + kx $, you get $\displaystyle dy/dx = 2x + k $. What you must do is compare this to $\displaystyle y = 4x + 9 $ in order to find k.