Pythagoras' theorem only works in a right-angled triangle. Find the $90^{\circ }$ angle first. The side opposite the right angle is the hypotenuse, which is always the longest side.

If the two shorter sides are $a$ and $b$, and the hypotenuse is $c$, then

$$a^2+b^2=c^2$$

This means the squares of the two shorter sides add to the square of the longest side.

Before you calculate, label the right angle and the hypotenuse. Also make sure every side length is in the same unit before you substitute into the formula.