Please help me. How do you do this.
 
											 
											Use the Pythagorean theorem:
[tex]x^2+4^2=10^2[/tex]
[tex]x^2+16=100[/tex] subtract 16 from both sides
[tex]x^2=84\to x=\sqrt{84}\\\\x=\sqrt{4\cdot21}\\\\x=\sqrt4\cdot\sqrt{21}\\\\\boxed{2\sqrt{21}}[/tex]