Apply Pythagoras theorem The side facing the right-angled is the hypothenus (c) AC2=32+42 a2=a×a AC2=3×3+4×4 AC2=9+16 AC2=25 A number that can multiply itself and give 25. 25=5×5=52 AC2=52 Since the powers are the same, the bases are also the same. AC=5cm