For a), all you need to do is replace the variable, n, for the number 13 (your 13 tons given) So, it will be C = 7500 + 175(13). 175(13) means 175 multiplied by 13 tons. Add 7500 to that and it will give you C, your cost.
For b), you replace your total cost, C, with $11,175, so it will be 11 175 = 7500 + 175n.
Then, it would be 11 175 - 7500 = 175n
(7500 becomes negative when you move it to the other side of the equal sign).
Then, 3675 = 175n
Then, 3675/175 = 175n/175 ( a / means division)
That leaves you with n = 21.
So, the answer for b) is 21 tons.