Question

Sistema

    x+y=3
12x-9y=20

Answer 1

y=3-x
12x-9(3-x)=20
12x-27-9x=20
3x=20+27
3x=47
x=47/3

Answer 2

x+y=3

x=3-y

12x-9y=20

12.(3-y)-9y=20

36-12y-9y=20

-12y-9y=20-36

-3y=-16  (-1)

3y=16

y=16/3