How much will it cost me to...

You know:

* 15 miles to the gallon (distance / fuel used).
* 50 miles (distance).
* \$5.80 per gallon (cost / fuel used).

You want to know how much fuel is used and how much that costs. There aren't any units to convert here, so the math is really straight forward.

* Do $Distance$ / $Distance / Fuel Used$ to get Fuel Used out.
* Multiply that by $Cost / Fuel Used$ to get the cost.

Does that help?
You have two jobs to do. 1- work out how much gas the trip is going to use. 2- once you know that, work out how much that gas is going to cost. Can you think how you might do either of those using the info you’ve got?