Changing a Quadratic from Standard Form to Vertex Form by MrsQuesnelle addresses how to complete the square to put an equation into vertex form when a does not equal 1.
This video called ❤² How to Solve By Completing the Square (mathbff) by mathbff explains how to solve by complete the square. It addresses when a = 1, a not equal to 1, and a = negative.
Note: this is not how you put the completing the square into vertex form.
