how do i use the calculator to find the slope of a line passing through the points(-4,0)and (2,-4)

I don't think the calculator can do that, but this website can :

http://www.wolframalpha.com/input/?i=slope+%281,4%29,+%285,3%29

Just type :     slope (-4, 0) , (2, -4)     and hit enter