How Does Substituting u=\alpha^2 Simplify Solving Quadratics?

[CyberShot]

http://imageshack.us/photo/my-images/809/equationu.png/
I'm not understand this simple step.
Btw, this isn't technically a homework question so I think it should be fine posting it here.

Thanks

 

[Char. Limit]

Let $u=\alpha^2$ and then solve for u via the quadratic formula. It'll come out plain as day.