Using grouping, factor 2 a2 – 13a + 21.

a.y = (2a + 7)(a + 3)
b.y = (2a + 7)(a − 3)
c.y = (2a – 7)(a + 3)
d.y = (2a – 7)(a − 3)

Commutative - ORDER doesn't matter. ex. 5+7 = 7+5

Associative - GROUPING of values doesn't matter. ex. (7 +/* 2) + 4 = 7 +/* (2 + 4)

Identity - Anything added to ZERO is the original number. Anything multiplied by 1 is the original number. ex. 4 + 0 = 4 * 1 = 4

Inverse - Using the OPPOSITE to cancel the terms. ex. a + (-a) = (-a) + a = 0    

a * 1/a = 1/a * a = 1  (if a ≠ 0)

Zero - Multiplying by ZERO, always equals ZERO.  ex. 4 * 0 = 0

Distributive - Distribute a value outside the parenthesis to all values inside the parenthesis. ex B(A+Y) = BA + BY

Hope this helps! :)