\[ f'(x) = \frac{d}{dx}(4x^2 + 16x + 3) \]
\[ f'(x) = 8x + 16 \]
\[ 8x + 16 = 0 \]
\[ 8x = -16 \]
\[ x = \frac{-16}{8} \]
\[ x = -2 \]
Ответ: -2.