solve by the substitution method
3x+5y=21
x=51-9y
substitute x in the eq 3x+5y=21
3(51-9y)+5y=21
153-27y+5y=21
153-22y=21
-22y=21-153
-22y=-132
-22y/-22=-153/-22
y=6
3x+5y=21
x=51-9y
3(51-9y)+5y=21
153-27y+5y=21
153-22y=21
-22y=21-153
-22y=-132
-22y/-22=-153/-22
y=6