Question 60824
IF YOU ARE SOLVING FOR Y THENYOU SUBSTITUTE 2X FOR Y THUS
X+2X=12
3X=12
X=12/3
X=4 SOLUTION
NOWSOLVE FOR Y BY SUBSTITUTING 4 FOR X THUS
4+Y=12
Y=12-4
Y=8 SOLUTION
PROOF 
X+Y=12
4+8=12
12=12