You can put this solution on YOUR website! Rearrange things so you have just x alone on the left.
Note that when moving things across you change the sign.
Start 18. -3x + 4y = 11
so -3x = 11 - 18 -4y
Simplify the right hand side
so -3x = -7 -4y
Multiply everything by -1 to get the x on the left side positive
so 3x = 7 + 4y
Divide both sides by 3
so x = (7 + 4y)/3
Best regards, Eric.