В прямоугольном треугольнике OKM:
tan(∠OMK) = OK / OM
OM = AM - AO = AM - MB = 12 - 4 = 8
tan(30°) = OK / 8
OK = 8 * tan(30°)
tan(30°) = \(\frac{\sqrt{3}}{3}\)
OK = 8 * \(\frac{\sqrt{3}}{3}\)
Ответ: OK = \(\frac{8\sqrt{3}}{3}\)