To see the slope and y-intercept more clearly, get y by itself.
3y=-15x
y=(-15/3)x
y=-5x
If you use the form y=mx+b, you can see that the slope is -5, and the y-intercept is 0. This means you will have a point at (0,0), and then you will go down 5, and across to the right 1, and so on.