Answer:
Completing the square
Step-by-step explanation:
The proof is lengthy, and I don't think you came here to see that whole proof. Here it is anyway to show you the process of completing the square.
https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:quadratic-functions-equations/x2f8bb11595b61c86:more-on-completing-square/a/quadratic-formula-proof-review
