a remote control airplane descends at a rate of two feet per second.After 3 seconds it is 67 feet above the ground. Write the equation in point slope form that models the situation. Then find the height of the plane after eight seconds

The equation modeling this problem is given by:
 y = mx + b
 Where,
 m: rate of change
 x: time in seconds
 b: initial height.
 We must find the constants m and b.
 For m we have:
 "descends at a rate of two feet per second":
 m = -2
 For b we have:
 "After 3 seconds it is 67 feet above the ground":
 y = -2x + b
 67 = -2 (3) + b
 67 = -6 + b
 67 + 6 = b
 b = 73
 The equation of the line is:
 y = -2x + 73
 For eight seconds we evaluate x = 8 in the function:
 y = -2 (8) +73
 y = -16 + 73
 y = 57 feet
 Answer:
 The equation in point slope form that models the situation is:
 y = -2x + 73
 The height of the plane after eight seconds is:
 y = 57 feet