\(y = x^{2} + 6x - 12\)

\(\frac{\mathrm d y}{\mathrm d x} = 2x + 6 = 0\)

\(2x = -6 \implies x = -3\)

\(y(-3) = (-3)^{2} + 6(-3) - 12 = 9 - 18 - 12 = -21\)