To identify the error in the procedure shown in the picture, it is important to remember the following:
Keeping this on mind, you can solve the given equation as following:
1. Given:
2. Square both sides of the equation:
3. Move the the term on the left side to the right side and add like terms:
4. Apply the Quadratic formula to find "x":
Therefore, you can identify that the error is in Step C, because: