Ascuba diver is 30 feet below the surface of the water 10 seconds after he entered the water and 100 feet below the surface after 40 seconds. what is the scuba divers rate of change

So 30:10 and 100:40

if we simplify them, and compare we get
3:1 and 5:2 which are not equal so therefor it is not a simple whole number rate of change so
slope is the sasme thing so
slope=(y1-y2)/(x1-x2)

lets says 10=x1
30=y1
40=x2
100=y2
subsitute
(30-100)/(10-40)=-70/-30=7/3
the rate of change is 7 feet per 3 seconds or 2 and 1/3 feet per second

I'm sure the answer is a