How do you get a slope if you only have the y-intercept.

Question

example: (0,5) and (-3, -4)

To find slope:

slope = (y2 - y1) / (x2 - x1)

With your values:
slope = (-4 - 5) / (-3 - 0) = -9/-3 = 3

I hope this will help and is what you were asking.