Solution: Let $ u = 2x + 1 $, so $ x = \fracu - 12 $. Substitute into $ f(u) $: $ f(u) = 4\left(\fracu - 12\right)^2 + 12\left(\fracu - 12\right) + 9 = (u - 1)^2 + 6(u - 1) + 9 = u^2 - 2u + 1 + 6u - 6 + 9 = u^2 + 4u + 4 $. Thus, $ f(x^2 - 3) = (x^2 - 3)^2 + 4(x^2 - 3) + 4 = x^4 - 6x^2 + 9 + 4x^2 - 12 + 4 = x^4 - 2x^2 + 1 $. \boxedx^4 - 2x^2 + 1 - Abu Waleed Tea