A = \sqrts(s - a)(s - b)(s - c) = \sqrt15(15 - 7)(15 - 10)(15 - 13) = \sqrt15 \cdot 8 \cdot 5 \cdot 2 = \sqrt1200 = 10\sqrt12 = 20\sqrt3 \, \textcm^2. - Crosslake