$\displaystyle y=- \frac{1}{3}x-1$\displaystyle y=- \frac{1}{3}x-1

$\displaystyle y= \frac{1}{3}x+1$\displaystyle y= \frac{1}{3}x+1

$y=3x+1$y=3x+1

$y=3x-1$y=3x-1