3x +4y =12
2x = y-6 Rewrite this as y = 2x+6
now replace that for y in the first equation
3x + 4(2x+6) = 12
now do all the calculations:
3x + 8x +24 =12
11x + 24 = 12
11x = -12
x = -12/11 = -1 1/11
now substitute -12/11 for x in the 2nd equation and solve
y = 2x+6
y = 2(-1 1/11) +6
y = 42/11 = 3 9/11