We can create two points.
30 feet below the surface of the water 10 seconds after he entered the water
100 feet below the surface after 40 seconds
So we have (10, 30) and (40, 100)
Plug this values into the slope formula:
m = y2-y1/x2-x1
m = (100 - 30)/(40 - 10)
m = 70/30
m = 2 1/3