set both values to equal each other
6x - 12 = 3x + 30
minus 3x from both sides to get
3x - 12 = 30
add 12 to both sides to get
3x=42
divide both sides by three to get x=14
since we know the x value is 14, substitute that for x in 6x - 12
6(14) - 12
multiple 14 and 6 to get 84 - 12
solve to gets 72
final answer: 72
hope this helped!