MathNet 2017-part-04 — 176 problems (EN–VI) Question 79

Let $\{a_n\}$ be a sequence such that $a_1 = 20$, $a_2 = 17$ and $a_n + 2 a_{n-2} = 3 a_{n-1}$. Determine the value of $a_{2017} - a_{2016}$.