As it is a right-angled triangle, you can use the Pythagoras theorem to work out the last side. The formula is A^2 x B^2 = C^2, where C is the hypotenuse (longest side).
5^2 is 25, and as you have two of those sides you double 25 to give you 50, which is C^2. To get C, you have to square root 50, which is 7.07106781187 cm or 7.1 cm to the nearest 10th.
