The square of the hypotenuse of a right-angled triangle is equal to the sum of the squares of the other two sides.
You can use Pythagoras' Theorem to calculate QR:
a^2 + b^2 = c^2
5^2 + 12^2 = QR^2
25 + 144 = QR^2
169 = QR^2
This means that the square root of 169cm is QR, which is 13cm.
SR is equal to QR, which means that SR is 13cm.
