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##...