FOR X: write 3x-y=10 in terms of y, add -3x from both sides, and divide both sides by -1 to simplify it down to y=3x-10. then, write 3x-10 in terms of y, add -3x to both sides, and divide both sides by -1 to get **x=10**

FOR Y: substitute 10 in terms of y and simplify to get **x=20**