[tex]\begin{gathered} y^2=15^2+x^2 \\ z^2=6^2+x^2 \\ \\ 21^2=y^2+z^2 \\ \\ 21^2=(15^2+x^2)+(6^2+x^2) \\ 441=225+36+2x^2 \\ 441=261+2x^2 \\ 2x^2=441-261 \\ 2x^2=180 \\ x^2=\frac{180}{2} \\ x^2=90 \\ \\ x=\sqrt[]{90} \\ x=3\sqrt[]{10} \end{gathered}[/tex][tex]\begin{gathered} y^2=15^2+x^2 \\ y^2=225+90 \\ y^2=315 \\ y=3\sqrt[]{35} \\ \\ \\ z^2=6^2+x^2 \\ z^2=36+90 \\ z^2=126 \\ \\ z=\sqrt[]{126} \\ \\ z=3\sqrt[]{14} \end{gathered}[/tex]
