Pythagoras equation for the hypotenuse of right angled triangles

I've got something which is really bugging me about Pythagoras, and I was wondering if you could help explain to me why the answer comes out as 21.26.

I have the equation shown here:

http://g.imagehost.org/0912/pythag.jpg

So I input (12,30) and (28,16) and the result should show as 21.26.

I am having trouble understanding how it has been worked out simply because whenever work it out, it seems to come out as 4.

So really my question is: How do I actually calculate using Pythagoras and these values above so that the answer comes out as 21.26?

Thanks if you can help.