If the hypotenuse of a triangle is 7cm and another side is 4cm, what's the length of the other side? How can I work this out?

The phenomenon you're describing is called 'Pythagoras theorem'. Let's go through it together. You have a right angled triangle, with the hypothesis and one side given. The pythagoras theorem gives you a formula you can use only for right angled traingles, and works regardless of what side is missing. It describes that the sum of the squares of the two sides connecting the right angle ('a' and 'b') is equal to the square of the hypotenuse (which we label 'c'). So, C^2 = a^2 + b^2

(Then I'd go through drawing two trangles next to each other, one with the letters a, b and c, and another with the example given. I would solve the first one, then I would get the student to answer the question itself using the format I have provided. If I see that he/she isn't understanding, I would give a simple example using numbers, but would ultimately let him/her answer their own question).