Okay first you change the first equation into one that gives you the value of x by taking
x + 3y = 3 and subtracting -3y from both sides which gives you
x = -3y +3 which you then substitute into the next equation
3y - 2(-3y +3) = 12 and then when you multiply you get
3y +6y -6 = 12 and when you’ve added like terms you get 9y -6 =12 adding +6 to both sides you get
9y =18 and then you solve and get
y=2 which you then substitute into the equation you’ve already solved for x ( x= -3y +3) which gives you x= -6+3 which means
y= 2 and x = -3
Hope this helps :)