Question

Please help me. How do you do this.

Answer 1

Use the Pythagorean theorem:

$x^2+4^2=10^2$

$x^2+16=100$        subtract 16 from both sides

$x^2=84\to x=\sqrt{84}\\\\x=\sqrt{4\cdot21}\\\\x=\sqrt4\cdot\sqrt{21}\\\\\boxed{2\sqrt{21}}$