Consider the equation
Using the extended Euclidean algorithm we find gcd(28,21) = 7 = 28 - 21. Moreover, it is easy to see that x = 2 and y = -2 is a solution of the equation. Hence all solutions are x = 2 - 21n/7 = 2 - 3n and y = -2 + 28n/7 = -2 + 4n, with n in Z.