Well, you know that you have 540,000 dollars to spend, so model x*18,000 + y*24,000 < 540,000.
Next, you know that at most, you may buy 25 new cars, so x + y < 25.
If you treat these two inequalities as equations, you can solve for the extreme points. That is say:
x+y = 25 so
y = 25 - x. Plug y into the first equation and you get 540,000 = x*18,000 + (25-x)*24,000.
Solve for x:
540 = 18x + 600 - 24x
-60 = -6x
x = 10
plug in x=10 into x+y = 25 and you get:
10 + y = 25
y = 15
So, if you buy 10 x model, and 15 y model cars, you'll spend all your money, and have the most cars possible. However, remember that these were inequalities. There is nothing stopping you from spending all your money on model x, or all of it on model y. Also, nothing says that you have to spend all your money either. If you spent all your money on model x, you would get 540,000 / 18,000 = 30 of those. If you spent it all on model y, you would get 22.5 of them. To show the region described by the system, I would make a two sided graph that looks like this:
| - .|
|_________| The left vertical line is model x, the right is model y. Going across the bottom is increasing model x and decreasing model y. The y-intercept on the left axis is 30, and the y-intercept on the right axis is 22.5. Where the two cross in the middle is where you get the most car for your money.
Best of luck!
Sorry, the formatting on the graph got messed up. It should be a horizontal line with two vertical lines sticking up at each end.