MathDB
Not too difficult.

Source: JbMO 2004, PROBLEM 3

July 20, 2004
quadraticsmodular arithmeticnumber theory proposednumber theory

Problem Statement

If the positive integers xx and yy are such that 3x+4y3x + 4y and 4x+3y4x + 3y are both perfect squares, prove that both xx and yy are both divisible with 77.