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/mathem...rs-647105.html

If the sequence is



The first question is simply applying n=10:



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