Let x be the number of used cars; y be the number of new cars. Since there are two unknowns, we have to formulate two independent equations to be able to solve the problem. Although, I think that would be 16,000, and not 1,600. It is only logical that new cars are more expensive than used cars.
(1) x + y = 17
(2) 7000x + 16000y = 191000
Solving these equations simultaneously:
x = 17 - y (1)
7000x + 16000y = 191000 (2)
7000(17-y) + 16000y = 191000
y = 8 new cars
Then, substituting this to equation 1,
x = 17 - 8 = 9 used cars
