Three would be your answer :)
You do not want the "b" since two of the equations start off at 0 kites.
x=kites purchased
Kities : y=8x
Windy Kites: y=20x
From there, all you need to do is plug in the kite purchased number replacing x.
Answer: 9+4n-1 = 20
We can solve this by substitution.
Replace n with the value given, 3 (remember 4n means 4 times n):
9 + 4*3 - 1
Then work it out using arithmetic
9+12-1
=20
