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For the composite function F(G(X)), just replace the function G(X) in the X of F(X):
F(X) = X² - 2X - 3
G(X) = 5X²
F(G(X)) = [G(X)]² - 2 · [G(X)] - 3
F(G(X)) = [5X²]² - 2 · [5X²] - 3
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