Ax+b=12.5 need an explanation

Step-by-step explanation:

Isolate X.

Ax-b=12.5

Ax=12.5+b. Move the other variable that can be added or subtracted first.

Now divide by A on both sides, and x is isolated.

x = (12.5 + b) / A

The parentheses are important, for they say that everything inside is divided by A, which is what you want. These problems look strange, because x will not turn out to be some number but a combination of variables. That is fine, so long as it is one X, and nothing else is on that side of the equality.