How can I use the Pythagoras' Theorem to work out the length of a missing side of a triangle?

We have the triangle ABC. AB = 3 BC = 4 AC = ?
By inputting these numbers into the Pythagoras' Theorem, we can out the length of the missing side (AC).
a² + b² = c²
AB² + BC² = AC²
9 + 16 = AC²
25 = AC²
Square root both sides of the equation and we get AC = 5