Answer:
Part A : The required Profit equation : 
Part B : 0.303 million radios the profit became 18 million.
Part C : The graph is attached below. 
Step-by-step explanation:
Given : The demand function for a type of portable radio is given by the model  , where d is measured in dollars and x is measured in millions of units. The production cost is $25.00 per radio.
, where d is measured in dollars and x is measured in millions of units. The production cost is $25.00 per radio.  
Note: Given that profit = revenue - costs
Revenue is demand times number sold. 
Costs are productions costs per number sold. 
To find : 
Part A: Write an equation giving profit as a function of x million radios sold.
Solution : Let x be the number of radios.
Revenue is demand times number sold


Let the cost of x number of radios is 25x.             
Profit = Revenue - Costs
 
 

The required Profit equation : 
   
Part B : The company currently produces 3 million radios and makes a profit of $18,000,000, but would like to scale back production. What lesser number of radios could the company produce to yield the same profit? 
Solution :  It is shown graphically which is attached in part c
When we scale back production we get that,
When we sold 0.303 million radios the profit became 18 million.
Part C : Give a graph for the profit and label the parts of the graph.
The graph is attached below which shows the profitable radios points.