you should subtract left sides and right sides in order to make y disappeared
and you will have the following:
y - y = (x - 6) - (3x - 8) => 0 = x - 6 - 3x - 8 => 0 = -2x - 14 =>
2x = -14 => x = -14/2 = -7
Now you can use -7 in the system and you will get y
y = -7 - 6 = -13
So your answers will be:
x=-7 and y=-13