APL (5554)

'The hypothenusehypotenuse of this right triangle is',3⍕.5*⍨+/⎕*2

Test:

      'The hypothenusehypotenuse of this right triangle is',3⍕.5*⍨+/⎕*2
⎕:
      9 10
The hypothenusehypotenuse of this right triangle is 13.454

Explanation:

• ⎕*2: raise the values in the input to the second power
• +/: take the sum
• .5*⍨: raise the result to the 0.5th power
• 3⍕: round to 3 decimal places