I need to solve this algebraically. I already know the answer, which is +/-2.2604 or (log(5/2 +/- (rt21)/2))/(log(2)). I just need to know HOW to get there.

The equation:

2^x + 2^(-x) = 5

I tried solving it using factoring/quadratics, but couldn't figure out how to set it up.