In most cases, you can do the inverse (opposite) operation on both sides of an equation to cancel out a certain value.
For example:
2x + 3y = 20
-2x + y = 4
This is a system of equations, just combine them to give you:
4y = 24
and divide both sides by 4:
y = 6
Sorry if this isn't what you were asking for, but the question was so vague, and this is what I assumed you were asking about, hopefully this helps regardless.
