C represents the total cost
S represents the amount of sugar
You have to write an equation for the cost with respect to the amount of sugar transported.
The total cost is equal to the sum of the rental cost for the trucks and the additional cost per ton of sugar transported.
You know that the rental cost of the trucks is $3500, this is a fixed cost, regardless of the amount of sugar.
The additional cost to transport the sugar is $125 per ton, so if you transport "S" tons of sugar, the cost will be $125S
You can express the equation for the total cost as follows:
Using this equation you can determine the cost of transporting 13tons of sugar, simply substitute the equation with S=13 and solve for C:
The cost for transporting 13tons of sugar is C= $5125