What laws do Probs. 12–16 illustrate?

Data from Prob. 12

(a + b) + c, a + (b + c)

Data from Prob. 13

b + c, c + b

Data from Prob. 14

3c - 6d, 3(c - 2d)

Data from Prob. 15

7(c - b), 7c - 7b

Data from Prob. 16

9/2 a - 3c, 9 (1/2 a - 1/3 c)