Equivalent substitution means that a quantity is replaced by an equivalent quantity, which is a basic thinking method in mathematics and the basis of algebraic thinking method. Substitution is a basic thinking method in mathematics and the basis of algebraic thinking method.

Properties of the equation: ① Add (or subtract) the same number or the same equal algebraic expression on both sides of the equation, and the result is still an equation.

② Multiply (or divide) two sides of the equation by the same number (divisor cannot be zero), and the result is still an equation.