Answer:
Step-by-step explanation:
Use Pythagorean theorem,
Base² + altitude² =hypotenuse²
x² + 19² = 21²
x² + 361 = 441
x² = 441 - 361
x² = 80
[tex]x =\sqrt{80}=\sqrt{2*2*2*2*5}=2*2*\sqrt{5}=4\sqrt{5}[/tex]
