Search Pythagorean Triples and use those to start off don't just pull funky numbers out of your butt. Make sure the student can easily find the square roots without a calculator to start. AFTER they understand the simple concept then you can start showing them that if we know ANY two sides we know the third side; at last we can put in any two numbers and find the third side, BUT START WITH PYTHAGOREAN TRIPLES.
Again the student can see that "C" isn't 25, it's the SQUARE ROOT of 25...they can also SEE that the two little squares have the same area as the big square...
Here is a screen cast talking about the above graphics...
Now go to the pre-calc page at the house of math and watch the screencasts so that we can apply this knowledge to find the distance between two points...and it should be simple to find the sides of triangles too...allow a few seconds for those screncasts to load...the page goes white for a few seconds and it seems like nothing is happening.
The House of Math on FaceBook.
The House of Math
Pythagorean Theorem at the House of Math.
Learn a lot of algebra for a buck.