... M-2 Origin of Algebra . ... 1) Isolate x in each equation and find its value, following the . rule that "when equal operations are performed on both . ... Write down a new equation for each step. ... For operations applied to both sides: (+2) add 2 . ... Solve once by (a) expressing y of one equation in terms of x, then . ... You may furthermore add one equation to the other, or subtract it: because the quantities you add or subtract to both sides are equal, what is left is also a valid equality. ...