Hey, I got most of the way through this reduction of order problem, but then my brain gave out right at the end.

Given the equation $\displaystyle y''-4y'+4y=0$ and the solution $\displaystyle y_1=e^{2x}$, write the general solution.

I make the substitution $\displaystyle y_2(x)=u(x)y_1(x)$ and do all the proper work and get here:

$\displaystyle e^{2x}u''=0$ and my brain gives out. I know this is easy, How do I find u?