The Addition Principle;
It is important to distinguish between equivalent expression and equivalent equations.
6a and 4a + 2a are equivalent expression because, for any replacement of a, both expressions represent the same number.

A equivalent equation is 3x = 15 and 4x = 20 a equivalent because and solution of one equation is also a solution of the other equation.
if we subtract 2 from both sides of the equation x + 2 = -5, we have x = 2 -2 = -5 -2, or x + 0=-7, or x=-7