Answer :
a+b=21
a=3+b
subsitute
3+b+b=21
subtract 3
b+b=18
2b=18
divide 2
b=9
there are 9 liters in b
a=3+b
subsitute
3+b+b=21
subtract 3
b+b=18
2b=18
divide 2
b=9
there are 9 liters in b