Math help! Just 1 question. Making an equation.

The fixed cost of a product is 4000 whereas its variable cost per item is 15.

Question: What is the expression for the total cost for x items?

so..
15x=4000?
4000/15=x?
or just 15x because the first two are equations and not expressions as 15x is.

15x=4000?

I dont understand the question