Well I am going to assume you need slope-intercept form. So the equation is y=mx+b. m is slope, b is y intercept. So you just plug it in -6= -1/2(6)+b. -6=-3+b. Now you figure out for b so what does b have to be to make the equation equal. Well -3 so -6=-3+-3, or -6=-6.
y=-1/2x-3
Hope that helped