a) Revenue = Price * Quantity. So, R(x) = x * p = x*(-20x + 910) = -20x^2 + 910x.
b) Profit = Revenue - Cost. So, P(x) = R(x) - C(x) = (-20x^2 + 910x) - (90x + 6000) = -20x^2 + 820x - 6000.
c) To find the value of x that maximizes profit, we need to find the vertex of the quadratic function -20x^2 + 820x - 6000. The x-coordinate of the vertex is given by -b/(2a), where a = -20 and b = 820. So, x = -820/(2*(-20)) = 20.5. Since this is a parabola with a negative coefficient of x^2, the vertex represents a maximum. Therefore, the value of x that maximizes profit is 20.5, and the maximum profit is P(20.5) = -20(20.5)^2 + 820(20.5) - 6000 = $8,405.
d) To find the price that should be charged in order to maximize profit, we need to find the corresponding value of p when x = 20.5. Using the demand equation p = -20x + 910, we get p = -20(20.5) + 910 = $310. Therefore, the price that should be charged in order to maximize profit is $310.