0.7b+0.9p = 21.2 is the equation.
You can buy 15 peaches, P = 15.
Plugging 11 in for b (equation above), you have spent $7.70 on bananas. Subtract this number from $21.20 to find that you have $13.50 left over for peaches. Divide 13.5 by 0.9 (since peaches cost 90 cents each), and your answer is 15.
With $21.20, you can buy 11 bananas and 15 peaches.