Let \(P(x) = x^4 - ax^3 + bx^2 - cx + 30 \) be with real values \(a,b,c\). Such that its positive real roots \(r_1, r_2, r_3, r_4 \) satisfy the equation \(6r_1 + 10r_2 + 15r_3 + 30r_4 = 120 \). What is the value of \( (c-b)^2 - a^2 \)?

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