If we arrive at an equation containing on each side the same term but with different coefficients, we must take equals from equals until we get one term equal to another term. But, if there are on one or on both sides negative terms, the deficiencies must be added on both bides until all the terms on both sides are positive. Then we must take equals from equals until one term is left on each side.

As quoted by Thomas Little Heath, Diophantos of Alexandria: A Study in the History of Greek Algebra (1885) - Arithmetica (c. 250 AD)