Every Pythagoras question is one of two cases, and you tell them apart by asking a single question: is the side I want the hypotenuse, or a shorter side? That answer decides whether you add or subtract.
Case One — Finding the Hypotenuse
You've got both legs and want the long slanted side. Square each leg, add them, square-root. Worked example: legs 5 and 12 → 5² + 12² = 25 + 144 = 169, then c = √169 = 13. Tidy, because 5-12-13 is a perfect set; most won't be, so you round to one decimal place — like √52 ≈ 7.2.
Case Two — Finding a Shorter Side
Now you know the hypotenuse and one leg, and want the other leg. You subtract: take the known leg's square away from the hypotenuse's square, then square-root. Worked example: hypotenuse 13, one leg 5 → a² = 13² − 5² = 169 − 25 = 144, then a = √144 = 12. Same theorem, rearranged.
One Method, Two Doors
It helps to write a² + b² = c² first every time, then plug in what you know and see which letter is missing. If c is missing you'll be adding; if a leg is missing you'll be subtracting. The toy throws both kinds at you so the “which one is this?” decision becomes instant — which matters, because mixing them up is the whole next rung.