How does Pythagoras Theorem work?

Pythagoras theorem works to find out an unknown side length of a right angles triangle (RAT) knowing the other two lengths. A RAT has 3 sides called a hypotenuse (longest side), and two smaller sides. We call these a, b and c. Pythagoras theorem tells us that a^2=b^2+c^2 and you can see this with a diagram of squares touching the sides of a triangle. a is the longest side, with b and c being the other two. You assign your letters and put the numbers of the two known lengths into the equation and re-arrange this to find your unknown length.