Ascuba diver is 30 feet below the surface of water 10 seconds after he entered the water and 100 feet below the surface after 40 seconds. find the rate of change including units.

Here, we can make 2 points

30 feet below the surface of water 10 seconds after he entered the water 
100 feet below the surface of water after 40 seconds

So, we have (10,30) and (40,100)

Slope = y2-y1 / x2-x1

So, lets substitute the values of the slope

m = (100-30)/ (40-10) 
    = 70/30
    = 7/3
m = 2 1/3  or  2.33