The problem set question may not give you the function directly, but instead, give you the slope of the demand curve \frac{dQ}{dP} and a point on the curve (usually the equilibrium quantity and equilibrium price). You will need to use the slope and the point on the curve to derive the market demand function. To do this, first recognize that the slope of the demand curve is the \frac{dQ}{dP}, which tells us that Q is on the left hand side of the expression. For instance, let us say that the question tells us that the slope of the demand curve \frac{dQ}{dP} is -10. Then the expression we have is as follows: Q=-10P+c, where c is the intercept. The question also gives us the equilibrium quantity and price (200, 20). Substituting (200, 20) into the expression, we have that 200=-10(20)+c. We can solve for c=400. Now, we need to rearrange the expression to get the price function for the firm P(Q), where P is a function of Q. Bring P to the left hand side of the equation and simplify the terms. The price function for the firm is hence P=40-\frac{Q}{10}. This is what you should input into the calculator.