The constat rate is given by the slope of the line. By definition, the slope of the line is defined as follows:
Where (X1,Y1) and (X2,Y2) are given points on the line. In our case, for example, we can take the points:
(X2,Y2) =(4,50)
(X1,Y1)=(2,70)
Replacing the previous data in the equation of the slope we obtain:
Then, we can conclude that the constant rate is
-10 ft /s : a descent of 10 ft per second.