This question can be solved using the Pythagorean theorem. [tex]a^2 + b^2 = c^2[/tex] where a and b are the legs of the triangle and c is the hypotenuse.
[tex]4^2 + 5^2 = c^2[/tex]
[tex]16 + 25 = c^2[/tex]
[tex]41 = c^2[/tex]
[tex]\sqrt{41} = c[/tex]
So, c is approximately 6.403.