Every right triangle follows the pythagorean theorem: given the legs [tex]a[/tex] and [tex]b[/tex] and the hypothenuse [tex]c[/tex] we have
[tex]a^2+b^2=c^2 \implies c=\sqrt{a^2+b^2}[/tex]
In your case, a=3 and b=5, so we have
[tex]c=\sqrt{3^2+5^2}=\sqrt{9+25}=\sqrt{34}[/tex]
