When you solve an equation, you use a process called deductive reasoning where you apply ideas that you know to be true. For example, you know that if you add the same number to both sides of an equation, the equation will not be changed. Many of the "ideas" that you use when solving are, in actuality, the mathematical properties (rules) that we saw in Real Numbers and Properties.

 Using real number properties (such as the commutative, associative, and distributive properties) and the properties of equality (such as adding, subtracting, multiplying and dividing by a non-zero)
justify why each step in the process of solving a linear equation is legal!

Solve for x and justify each step with a reason: 3(x - 2) + 5x = 9x - 24
 Steps: Justification (Reasons): 3(x - 2) + 5x = 9x - 24 Given 3x - 6 + 5x = 9x - 24 Distributive Property 3x + 5x - 6 = 9x - 24 Commutative Property of Addition 8x - 6 = 9x - 24 Combine Like Terms 8x - 8x - 6 = 9x - 8x - 24 Subtraction Property of Equality 0 - 6 = x - 24 Additive Inverse Property (left) Combine Like Terms (right) -6 = x - 24 Additive Identity Property -6 + 24 = x - 24 + 24 Addition Property of Equality 18 = x + 0 Addition (left) Additive Inverse Property (right) 18 = x Additive Identity Property

The justification method shown above is an example of one method.
There are other justification methods that are deemed acceptable.