First, you have to work out whether the side you need to find is the hypotenuse or not. The hypotenuse is the longest side of the triangle, opposite the right angle. 

For this question, you use Pythagoras' theorem:

a² + b² = c² (Where c is the hypotenuse and a and b are the other sides of the triangle). 

For example, if a triangle has its two sides adjacent to the right angle as 3cm and 4cm, the square of the length of the third side is 3² + 4² = 9 + 16 = 25

Therefore, the length of the final side is the square root of 25, 5cm. 

If you are given the length of the hypotenuse and one other side as 10cm and 6cm respectively, you can find the third side by rearranging the equation: 

c² - a² = b²

10² - 6² = 100 - 36 = 64 = 8²

^{b = 8.}