How do i solve these 2 NP complete problems?

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A1, A2, A3 ⊂ A in pairs of 2 they have NO common spot. Their union is A and they have same sum. How can i see this is NP complete .

Second Problem. A = {a1, . . . , an} How do i see if a, b, c, d ∈ A different between them exists so a + b = c + d?