Does anyone know how to turn a value into a negative number?

An example:

that would make x be the negative value of b/2a [incase you don't know, it's 1/2 of a quadratic function].Code:x = -(b/2(a))

And I know if b/2a comes out negative, the - sign makes it a positive, but I wish to know how to make it negative if it isn't already.