Hi, please help me on this equation

1) A sequence is defined by Un+1=Un+2Un-1
a) Given that U11=683 and U8=85 deduce the equations
U10+2U9=683
U10- U9=170


b) Find the value of U9

This is a continuation of the sam question you asked earlier here: https://www.askmehelpdesk.com/mathematics/sequence-series-questions-answers-647105.html

If the sequence is


U_{n+1} = U_{n} + U{n-1}


The first question is simply applying n=10:


U_{(10+1)} = U_{10} + 2 U_{(10-1)}, \\
U_{11} = U_{10} + 2 U_9


For the second use n=9, and rearrange to get the result.