Explanation:
no wizardry is needed, as the function is already there. we only need to use it.
(a)
find d, so that 162 - 6d = 0 (0 miles are left)
162 - 6d = 0
162 = 6d
d = 162/6 = 27
it will take them 27 days.
(b)
L(17) = 162 - 6×17 = 162 - 102 = 60 miles
after 17 days they still have to lay 60 miles.