Chapter 13, Trigonometric Ratios and Functions - 13.6 Apply the Law of Cosines - 13.6 Exercises - Skill Practice - Page 892: 27

$$A=\sqrt{17391}$$

By Heron's Formula, we know: $$A=\sqrt{s(s-a)(s-b)(s-c)}$$ Where: $$s=\frac{a+b+c}{2}$$ Thus, we find: $$s=\frac{14+20+28}{2}=31 \\ A=\sqrt{31\left(31-14\right)\left(31-20\right)\left(31-28\right)} \\ A=\sqrt{17391}$$