How do you find the length of the longest side of a right-angled triangle?

First of all, I would ask the student what they think the question means and whether they have encountered a question like this before, so that I know where to start from. If the student is more of a visual learner, I'll draw a right-angled triangle, after which I'll explain the basis for Pythagoras' Theorem and to fully illustrate my point I'll create some real life scenarios where calculating the length is applicable. I'll then move onto more calculations and use simple examples to build up their confidence in the topic until they are capable of answering questions independently.