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See the diagram
let O be the center of the circle
therefore, AE is the diameter
AE = AD + DE
AE = 5 + 3
AE = 8 cm
AO = r
AO = 8 ÷ 2
AO = 4 cm
In triangle AOC
AO = OC = 4 cm                 (radii)
AO = OC = AC = 4 cm            (given AC = 4cm)
Triangle AOC is an equilateral traingle

In triangle AOB
AO = OB = 4 cm                      (radii)
angle OAB = angle OBA          (angles opp. to equal sides are equal) 
angle OBA = angle OAB = angle OAC = angle OCA    (angle OAB = angle                                                  OAC given, angle OAB = angle OBA, angle OAC                                              = angle OCA)
In traingles AOB and AOC
AO is common
angle OAB = angle OAC          (given)
angle OBA = angle OCA          (proved above)
therefore by AAS congruence rule
Triangle AOB = Triangle AOC
therefore by CPCT
AB = AC 
AB = 4 cm
please mark the best
3 4 3