12)
3a + 5 = 2a - 6
3a = 2a - 11
a= - 11
13)
a - b = 9
a + b = 55
rewrite the second equation
a = 55 - b
substitute this new equation into the first equation
(55 - b) - b =9
55 - 2b = 9
-2b = -46
b = 23
then solve for a
a - (23) = 9
a = 32
