You have the following two expressions:
a) 14x + 21
b) 6 - 3x
In order to find the factor for each expression, you consider that the factor is a term which multiplied by the terms inside the parenthesis generates the original expression. The factor is deduced from the estimation of the least common multiple of the terms.
a) 14x + 21
you have that least common multiple is 7. Then, you have:
14x + 21 = 7(2x + 3)
next, you use the distribution property in the expression 7(2x + 3)
7(2x + 3) = (7)(2x) + (7)(3) = 14x + 21
thus, you obtain the original expression and 7 is in fact, the searched factor.
b) 6 - 3x
the least common multiple of this expression is 3. Then, you have:
6 - 3x = 3(2 - x)
next, you use distribution property
3( 2 - x ) = (3)(2) + (3)(-x) = 6 - 3x
thus, the factor is 3