Pythagoras
Square the two sides you know, add or take away, then square root. Every triangle here has whole-number sides, so the arithmetic never gets in the way of the idea.
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It is the squares that add up, not the sides
3 + 4 is 7, and the long side of a 3-4-5 triangle is 5. The sides do not add — the SQUARES do: 9 + 16 = 25, and 25 is the square on the long side, so the side itself is √25 = 5. Skipping that last square root is the other common slip.
Which one is the hypotenuse?
It is the side opposite the right angle, and it is always the longest. Not the slanted one, and not the one on the right — the triangles here are drawn in four different orientations on purpose, because a rule you can only apply to one picture is not a rule yet.
Going backwards
If the missing side is one of the SHORT ones, the answer has to come out smaller than the longest side. That means taking away rather than adding: square the long side, take off the square of the side you know, and square root what is left.
School levels
School levels are a guide only. They show where a skill is usually taught in each country and are not taken from, aligned to, or endorsed by any curriculum authority. Your school may cover it earlier or later.