Maths problem: how to do (b), thanks?

(a)
AB² = AC² + BC² (畢氏定理)
AB² = 4² + 3² = 16 + 9 = 25
AB = 5 cm

(b)
CD = AE = 5 cm
BD = BC + CD = 3 + 5 = 8 cm
BE² = BD² + ED² (畢氏定理)
BE² = 8² + 4² = 64 + 16 = 80
BE = √80 = √(16 × 5) = 4√5 cm