Answer:
a=12 b=9
Step-by-step explanation:
u can substitute 3/4a in for b in a+b=12 since it is equal to b, giving you
a+3/4a=21
a=4/4a
4/4a+3/4a=21
7/4a=21
4/7(7/4a=21)
a=12
plug in 12 for a in either equation
b=3/4*12
b=9
(21=12+b) -12
9=b
