A manufacturer produces bolts of a fabric with a fixed width. A quantity q of this fabric (measured in yards) that is sold is a function of the selling price p (in dollars per yard), so we can write q=f(p). Then, the total revenue earned with selling price p is R(p)=pf(p).

Find R(30), given f(30)=19000, and f(30)=250.

I've tried substitution and attempted to derive it as well but I feel that I am still very lost as to how to tackle this problem.