I'm not sure I understand completely, but as I understand it:

You are trying to solve this equation for b:

By using the vertex-form of a quadratic?

There are a few things I don't understand:

-Where has the variable

come from?

-How have you deduced that

? There is no reference at all to a variable '

' in the original equation.

If I had to *guess* I'd say that your "values" for a,b,c refer to the form

of a quadratic, although you shouldn't be using "

" here to mean two different things so I'm going to call it

Sorry, I accidently put a d instead of a b xD

We have:

because in this equation, b is the variable we are trying to solve for,

**not** x. I don't understand at all which method you are trying to communicate, but

**hopefully** this start will help.