The Goal

Solve the following equation for y:

The Mistake

Find the algebra mistake:

Need a hint? Look carefully at the red part of the algebra:

The Correction

An Explanation

There are three mistakes, all sign errors. In the first step the sign of c is changed for no reason. In the second step the sign of the x² term is switched. In the third step the student forgot the ± in front of the square root.
Take great care with signs!