Area

A trivial application of Heron's formula . Let $s=\frac{a+b+c}{2}$ be the semiperimeter of the triangle. The area is then given by $A=\sqrt{s(s-a)(s-b)(s-c)}=\sqrt{21(21-13)(21-14)(21-15)}=\sqrt{21\cdot 8\cdot 7\cdot 6}$

Making further simplifications, $A=\sqrt{7^2\cdot 3^2\cdot 2^4}=7\cdot 3\cdot 4=\boxed{84}$