Simplify:

Factor the numerator: 3x + 6 = 3(x + 2)

Factor the denominator: 2x + 4 = 2(x + 2) 
Rewrite the expression in terms of the factors:

The common factors in the numerator and denominator cancel out because any expression divided by itself is 1; therefore,
Example

Factor the numerator: x - 24 = 4(x 6)

Factor the denominator: x² - 9x + 18 = (x 6)(x 3)

Rewrite the expression in terms of the factors: =

Identify the common factors in the numerator and the denominator. These two factors cancel out because any expression divided by itself is 1.