To find the slope of the line passing through the points (5, -1) and (4, -5), you would need to use your slope intercept formula, (y2-y1)/(x2-x1) which calculates your slope. When you plug the x and y values of your coordinate into your formula, you should get:
(-5-(-1))/(4-5)
When this is simplified, you should get -4/-1, or when simplified further, you should get that your slope is 4.