This is the question,
I understand the concept, in ##\mathbb{Z_n}## an element is a is a unit if and only if gcd( a,n) =1.
My understanding of backwards substitution,
...
i have using Euclidean algorithm,
##471 = 3⋅121 + 108##
##121 = 1⋅108 + 13##
##108 =8⋅13+4##
##13=3⋅4+1##
##4=4⋅1+0##
using back-substitution,
##1=13-3⋅4##
##=(121-1⋅108)-3(108-8⋅13)##
...
##= 121-(471-3⋅121)-3⋅471+9⋅121+24⋅121-24(471-3⋅121##
##=121-471+3⋅121-3⋅471+9⋅121+24⋅121-24⋅471+72⋅121##...