Riley is 3 years older than his sister. the sum of their ages if s represents riley's sister's age is shown as
a. 3s
b. b s + 3
c. 2(s + 3)
d. 2s + 3

try using substitution:

x + y - 6 = x - y. (i'm not giving you the exact answer). since 0 = x - y, then sub 0 in the equation with x - y. combine your like terms, and then use inverse operations, and you'll have your answer.

: -)

a. to encourage apoptosis