x=1, y=1 looks a good guess which turns out to be correct. Add on half of the first equation to itself (so we get another "3x" equation) to get -3x-15y=-18. Now add the 2 equations to get rid of the 3x, giving you -9y=-9 or y=1. Use y=1 in either equation to get x=1.
In general you can turn one equation around to give x in terms of everything else. The substitute your expression for x into the other equation which will give you an simple equation for y only.
The method I used above found an expression for 3x rather than x alone because that's easier to substitute into the other equation.
2x-12
3x+8 = 12 3x = 4 x = 4/3
3x+42=6x+12 42=6x-3x+12 42=3x+12 42-12=3x 30=3x 10=x
Point slope equation: Y=3X+12
7x = 3x - 12 4x=-12 x=-3
2x-12
3x+8 = 12 3x = 4 x = 4/3
3x+42=6x+12 42=6x-3x+12 42=3x+12 42-12=3x 30=3x 10=x
Point slope equation: Y=3X+12
3x+12=x+48: 3x-x=-12+48 2x=36 x=18
3x - 12 = -2x + 235x = 35 so x = 7
7x = 3x - 12 4x=-12 x=-3
No.
y-3x+6y=12 7y=3x+12 y=3x/7 +12/7
6x+4=3x+12 => 6x-3x=12-4 => 3x=8 => x=8/3=2.666...
y=3x+12
70000000y=35t