To write the given equation in function form consists in solving the equation for y:
a)
x - 3y = -9 subtract x both sides
x - x - 3y = -9 - y eliminate x terms left side
-3y = -x -9 divide by -3 both sides
y = 1/3 x + 3
b)
3x + 4y = 0 subtract 3x both sides
3x - 3x + 4y = -3x eliminate x terms left side
4y = -3x divide by 4 both sides
y = -3/4 x
c)
y - x = -1 add x both sides
y - x + x = -1 + x eliminate x terms
y = x - 1