13) $(3 x+8)\left(4 x^{2}-4 x+7\right)$

Step 1 ：Expand the expression: \((3x + 8)(4x^2 - 4x + 7) = 3x(4x^2 - 4x + 7) + 8(4x^2 - 4x + 7)\)

Step 2 ：Distribute the terms: \(12x^3 - 12x^2 + 21x + 32x^2 - 32x + 56\)

Step 3 ：Combine like terms: \(12x^3 + 20x^2 - 11x + 56\)

Step 4 ：\(\boxed{12x^3 + 20x^2 - 11x + 56}\)