f(x) = 2x^3 + a(a+3)x^2 - 2x + (2b-a)
f(0) = 2b -a > 0 => 2b > a
f(1) = f(-1)
= a(a+3) + (2b-a)
= a^2 + 2a + 2b
= 0
=> 2b = -a^2 - 2a > a
=> 0 > a(a + 3)
=> -3 < a < 0
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