I'm not sure what your question is but let me give you a little math lesson. when you have a y intercept, the number given is your y coordinate, that is at what point on the y axis your line is crossing. so on this line we have the point (0,12) we know this because the line crosses on the y axis so therefore our x value has to be zero.
Because they have given us the slope you simply start at an already known coordinate. (0,12) and use the old rise over run rule. because the slope is negative you know your line is going to look like this \ so start at 12 and go down four. (0,8) then to the right one. (1,8) and that would be another point on your line.
