Use the given information as (x, y) points.
You have
(1, 34)
(2, 50)
(3, 66)
(4, 82)
The difference between renting for 1 day and 2 days is 50 - 34 = 16.
The difference between renting for 2 days and 3 days is 66 - 50 = 16
The difference between renting for 3 days and 4 days is 82 - 66 = 16
For each day you rent you pay $16.
Now look at renting for 1 day.
The rental for 1 day is $16, but for 1 day you pay $34.
$34 - $16 = $18
There is an extra $18 in the rental for every day. The $18 is fixed.
That means, to rent a truck, you must pay a fixed amount of $18 plus $16 per day.
total cost = daily cost + fixed cost
total cost = 16d + 16
c = 16d + 18