The standard from of a line has both variables on one side of the equal sign and a constant on the other side. (by convention is should be arranged so that the coefficient of x is positive)
y-3=3(x+1) perform indicated multiplication on right side
y-3=3x+3 subtract 3x from both sides
y-3x-3=3 add 3 to both sides
y-3x=6 multiply both sides by -1
3x-y=-6
