How to do the question 4 and question 5, want a clear step

. Suppose that the demand and supply functions for a certain model of a youth wrist watch are related by p : D(q) : 20 1.5g and p : S(q) = 1.25q, Where p is the price (in dollars) and q is the quantity demanded (in hundreds). . Joanne Wendelken sells silkscreened T shirts at community festivals and crafts fairs. Her marginal cost to produce one Tshirt is $3.50. Her total cost to produce 60 Tshirts is $300. (a) Find the linear cost function for Joanne's Tshirt production (b) What is the xed cost