With the data you give, the answer is ($0.50 food cost)(1.75) = $ 0.88
However, in the real world, french fries have other costs as well as food costs: labor, oil, electricity, container for the fries, supervisor & building overhead, etc. Now maybe all these non-food costs are "accounted for" by taking a high 75% margin to cover them and produce a profit.
If however, management still wants a 75% margin after allocating all costs to the fries, the the formula would be:
($0.50 food cost + C "other allocated costs") (1.75) = ? depending on the value C.