The following was confusing me, so any help/explanations would be greatly appreciated!
Find all incongruent solutions to the following linear congruence:
21x ≡ 14 (mod 91)
$\displaystyle \gcd(21,91)=7$ and $\displaystyle 7|14$ so there shall be $\displaystyle 7$ solutions in each congruence class. By inspection we see that $\displaystyle x\equiv 5$ is one equivalence class which solves this. By theorem it means $\displaystyle 5 + \frac{91}{7}t$ for $\displaystyle t=0,1,...,6$ shall all be solutions.
Yes.
For example $\displaystyle x\equiv 2 (\bmod 4)$ the solutions $\displaystyle x=2,6$ are really the same ("congruent") solutions because they are contained in the same equivalence class (equivalence class mod $\displaystyle 4$*)
*)The congruence classes are $\displaystyle \{...,-4,0,4,...\} , \{...,-3,1,5,...\}, \{...,-2,2,6,...\}, \{...,-1,3,7,...\}$. So $\displaystyle x=2,6$ are in the same equivalence class.