In this case, we are given a function L(d) = 135 - 5d that models the miles of track that are left after d days, by replacing 19 for d into the given function, we can determine how many miles of track does the crew have left to lay after 19 days, like this:
L(19) = 135 - 5(19) = 135 - 95 = 40
(a) Then, the team have left to lay 40 miles of track
Now we need to determine how many days will it take the crew to lay all the track, if the team finished laying all the track the team will not have left any miles, and then L(d) = 0, then we can calculate the day this happens like this:
L(d) = 0
135 - 5(d) = 0
Adding 5d on both sides:
135 - 5d + 5d = 5d
135 = 5d
Dividing both sides by 5:
135/5 = 5d/5
27 = d
d = 27
(b) Then, it will take 27 days for the team to lay all the track