first isolate y by adding x to both sides in y-x=3 and getting y=x+3
Step 1: substitute x+3 for y in 3y=x+6 to get 3(x+3)=x+6
Step 2: distribute 3x+9=x+6
Step 3: subtract like terms 2x=-3
Step 4: divide x=-3/2
substitute -3/2 for x in y=x+3 to get
y=-3/2+3
y=3/2
answer: y=3/2 x=-3/2