Answer
$$A \approx 88{{\text{m}}^2}$$
Work Step by Step
$$\eqalign{
& a = 15.8{\text{m}},\,\,\,b = {\text{21}}{\text{.7m,}}\,\,\,c = 10.9{\text{m}} \cr
& {\text{First}},{\text{ find the semiperimeter }}s \cr
& s = \frac{1}{2}\left( {a + b + c} \right) \cr
& s = \frac{1}{2}\left( {15.8 + 21.7 + 10.9} \right) \cr
& s = 24.2 \cr
& \cr
& {\text{Now use Heron's formula to find the area }}A \cr
& A = \sqrt {s\left( {s - a} \right)\left( {s - b} \right)\left( {s - c} \right)} \cr
& A = \sqrt {24.2\left( {24.2 - 15.8} \right)\left( {24.2 - 21.7} \right)\left( {24.2 - 10.9} \right)} \cr
& {\text{Use a calculator}} \cr
& A \approx 88{{\text{m}}^2} \cr} $$
