\(\angle \,ACH = {180^ \circ } — \angle \,ACB = {180^ \circ } — {120^ \circ } = {60^ \circ }.\)
По определению синуса из треугольника АСН:
\(\sin ACH = \frac{{AH}}{{AC}}\,\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\,\sin {60^ \circ } = \frac{{AH}}{{2\sqrt 3 }}\,\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\,\,\frac{{\sqrt 3 }}{2} = \frac{{AH}}{{2\sqrt 3 }}\,\,\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\,\,AH = 3.\)
Ответ: 3.