Since y is the variable, x is considered here as a constant.
Then :
-x + 3y = -6
we add x in both left side and right side of the equation :
-x + x + 3y = -6 + x
3y = -6 + x
Then we divide both side by three :
3y / 3 = (-6+x) /3
y = (-6+x)/3
y = -6/3 + x/3
y= -2 + x/3
