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.
Think about this, f(x) -> f(3). Look at the chart, x is 3. What number is directly to the right of 3. Your answer is 9.
7 is the correct answer .........................