This is a middle school problem, so I am not going to do the trigonomic way. For this you just use the Pythagorean Theorem (a^2+b^2=c^2)
So: 4^2 + 9^2= c^2
16 + 81= c^2
97=c^2
sqrt(97)
c=9.8
The length of the hypotenuse is 9.8 or 10.
But you want it in simpliest radical form so it is:
sqrt(97)