What values of x would make the following equation true?

There are many ways to do this. Such ways are using the quadratic formula, completing the square, etc.
We have  We can add 46 to both sides so the -21 becomes a +25. by doing that, we get  This can be wrote as  We solve the square root and get  (The +- is a plus minus sign.) Subtract five on both sides to get 
The answer is C.

Do what step by step?