You use the pythagoras theorem to find the lenght of the hypotenuse:
[tex]\begin{gathered} h^2=l^2+l^2 \\ h=\sqrt[]{l^2+l^2} \end{gathered}[/tex]
You have the measure of the two legs: 40 and 9:
[tex]\begin{gathered} h=\sqrt[]{40^2+9^2} \\ h=\sqrt[]{1600+81} \\ h=\sqrt[]{1681} \\ h=41 \end{gathered}[/tex]The hypotenuse is 41 units.
