Take two points
[tex]\\ \sf\longmapsto m=\dfrac{-3-0}{0+3}=\dfrac{-3}{3}=-1[/tex]
Equation of line in slope intercept form
[tex]\\ \sf\longmapsto y=mx+b[/tex]
[tex]\\ \sf\longmapsto y=-x-3[/tex]
