Let each of the unknown side be x. \(10^{2} = x^{2} + x^{2} - 2(x)(x) \cos 120\) \(100 = 2x^{2} - 2x^{2} \cos 120\) \(100 = 2x^{2} + x^{2} = 3x^{2}\) x = \(\sqrt{\frac{100}{3}}\) = \(\frac{10}{\sqrt{3}}\) x \(\frac{\sqrt{3}}{\sqrt{3}}\) x = \(\frac{10\sqrt{3}}{3}\)cm