I personally use y2-y1 over x2-x1 because it's simple. Plot two y values and two x values. The first ones you plotted are x1 and y1, the others are x2 y2. Now subtract y2-y1 and x2-x1. Afterwards, divide the y value by the x value, that will give you slope.
Answer: 80%
Explanation:
From the provided graph, read the Relative Cumulative Frequency as 0.8 (or 80%) which corresponds to a GPA of 3.5.
This number represents all GPAs lower than 3.5.