Since the room is square, we know that all four sides will be the same length. To find the length of one side of the room, we just need to take the square root of 169. In doing so, we get a final answer of 13.
A=LWH
divide both sides by LW
A/(LW)=H
42 = -6 + 3x
42 = 3x - 6 (just moving 3x and -6 around)
42+ 6 = 3x - 6 + 6 (adding 6 to both sides)
42 = 3x
42/3 = 3x/3 (dividing by 3 to both sides to isolate for x)
14 = x
You're trying to isolate for x, so you have to get it alone on one side. To do this you have to add/subtract and multiply/divide, but remember that whatever you do to one side you have to do to the other. Hope this helped! :)
Multiply 2x-8y=10 by 3 and that should eliminate the X's