Question

quanto e x-y=15+3y
x-3y=21+3y

Answer 1

x-y=15+3y
x-3y=21+3y

x-y-3y=15
x-3y-3y=21

x-4y=15    (-1)
x-6y=21

-x+4y=-15
x-6y=21
-2y=6
Y=6/-2
Y=-3 

X-4(-3)=15
x+12=15
x=15-12
x=3 


S=[X=3 Y=-3]