[tex]\begin{gathered} h=\sqrt[]{(20\sqrt[]{21})^2-(84)^2}_{} \\ h=8\sqrt[]{21} \\ \end{gathered}[/tex][tex]\begin{gathered} \frac{x}{8\sqrt[]{21}}=\frac{8\sqrt[]{21}}{84} \\ x=\frac{(8\sqrt[]{21})^2}{84} \\ x=\frac{1344}{84} \\ x=16 \end{gathered}[/tex]
