The function $f (n) = n^2 + n + 17$ for $0 \leq n \leq 15$ generates prime numbers. What is the value of $f (10)-f (9)$?

f(10) - f(9) = 28.

$f(n) = n^2 + n + 17$

$f(10) = 10^2 + 10 + 17 = 100 + 10 + 17 = 127$

$f(9) = 9^2 + 9 + 17 = 81 + 9 + 17 = 107$

$f(10) - f(9) = 127 - 107 = 20$