Which equation has a slope of -1/2 and a y intercept of -3?

a. 4x+2y+3=0
b. 3x+6y+6=0
c. 5x +10y+30=0
d. 6x+2y+1=0

I have the answer. I know it is C, but this was today's lesson and I still don't understand why or how to get C. Please help.