Commenter jdoe said it right: solve for y and leave the rest on the other side.
-x + 3y = 6
3y = 6 + x add x on both sides
3y = x + 6 rearrange to get the x first
y = (x + 6) /3 divide both sides by 3
y = x/3 + 6/3 split the numerator (caution - never split denominators)
y = x/3 + 2 simplify 6/3
Thus the line in slope intercept form of y = mx + b is y =