It's basic algebra; an expression is different than an equation.
"An equation contains an equal sign, expressions do not. An expression, even one that contains variables, represents a value. Even if you don't know that value, nothing you do to an expression can change its value." mgmat
"An equation contains an equal sign, expressions do not. An expression, even one that contains variables, represents a value. Even if you don't know that value, nothing you do to an expression can change its value." mgmat