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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