@chijioke
The way this question has been presented, I'm not terribly sure I know what it is asking you to do.
The only thing I can think of is that they want you to say how the profit made varies when the cost prices of the two cars vary.
I have set up a little spreadsheet that illustrates this and a picture of the results is shown below (qv).
In my spreadsheet, CP stands for Cost Price, SP stands for Selling Price and Car A is the one the dealer makes the 20% profit on (Car B, therefore, is the one with the 20% loss).
Points to note:-
1. The highest price that Car A can be is N250 000 because Car B then has a zero Cost Price and the net profit is then just the 20% made on Car A but that's an unlikely scenario (since Car B probably cannot have zero cost price?) So the Maximum profit is ≤ 20% (though, as I say, 20% is unrealistic).
2. The profit reduces as the cost price of Car A drops below N250 000.
3. Until the cost price of both cars is N150 000 (ie: the same) then there is zero profit made.
4. Once the cost price of Car A is below N150 000, then the profit becomes negative, ie: there is a net loss
5. The maximum loss (-20%) occurs when Car A has zero cost price but, again, this is an unlikely scenario.
All I can think of is that the question expects you to produce a formula (in terms of your m & n?) that will quantify this relationship (between the cost prices of the two cars and the net profit realized as those prices vary).
Do you think you could do that?
Have a go and let us see your attempt then further advice will be offered if necessary.
Hope that helps. 
Spreadsheet Output:-
View attachment 37848
PS: I have attached my spreadsheet ("# Car Sales.txt") so that you can download it and play around with it if you wish but, if you do download it, you must immediately change its filename from "# Car Sales.txt" to "# Car Sales.xlsx" so that it may be opened as a spreadsheet.