Simple...
you have: x-3y=18
You want to convert it into slope-intercept form or y=mx+b form-->>>
x-3y=18
Isolate the variable-->>
x-3y=18
-x -x
-3y=-x+18
Now simply divide...
\(\frac{-3y}{-3} = \frac{-x+18}{-3}\)
y=\(\frac{1}{3}x -6\)
Thus, your answer.