I am working on a class project on the derivation of the quadratic formula, and i understand the process of completing the square, up until the very last process of simplifying back to the quadratic forumla...

Can someone explain how

$\displaystyle

-\frac{c}{a} + \frac{b^2}{4a^2} = \frac{b^2 - 4ac}{4a^2}

$