Plz can you with part b and part c because am really confused

You could have done part one simpler.
Remember that since it is 20% all you have to do is multiply 5750 by 5 = 28,750. 
B) Remember that she gave a down payment of 5750 and according to the problem the rest of the money will be financed, so 28750 - 5750 = 23000.
C) Now we know that to find the remaining amount of money, you know that what was left to finance was 23000, but the price is increased by 30% so 
23000 * .3 = 6900
so add this to 23000 which equals 29900.

The exact length of the third side is 10