thru (4,5); m = -2/3
Start with slope-intercept form: y=mx+b
$\displaystyle y=\frac{-2}{3}x+b$
Plug your x and y coordinates in...
$\displaystyle 5=\frac{-2}{3}4+b$
$\displaystyle \frac{15}{3}=\frac{-8}{3}+b$
$\displaystyle b=\frac{23}{3}$
$\displaystyle y=\frac{-2}{3}x+\frac{23}{3}$
EDIT: Whoops, didn't see janvdl's post!