Proof. Suppose that there are non-negative integers x,y and w such that 7x + 8y = w4. Let z = w2. Then 7x + 8y = z2. By Theorem 3.1, we have (x,y,z) = (0,1,3). Then w2 = z = 3. This is a contradiction. Hence, the equation 7x + 8y = w4 has no non-negative integer solution