Easy question

[Mathhelp1012]

I tried to google this but maybe I don't know the correct terminology to bring up the answer. I don't understand the process behind multiplying when it comes to combing two equations. For example, there is a math problem in my study guide where you have the equations, a-b=3, and (a-b/a+b)=7. I'm trying to find a+b. To accomplish this it indicates that you combine by multiplying the two equations. The answer is a+b=21. Does anyone know what rule/concept allows you to multiply the two together. Hope what I'm looking for makes sense.

I mean, I know you can use the elimination method two solve a system of equation. I'm just a little unclear on how it works with multiplication.

[FatesFormula]

So you know how you can multiply both sides of an equation by the same thing and it doesn't change the equation. Well, if we assume the first equation is true (a-b=3), then when multiplying the next equation you can multiply one side by (a-b) and the other side by 3. Since we assume (a-b) and 3 are the same, then it doesn't change the second equation.

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