Answer: 10.25
Step-by-step explanation:
Use Pythagoras equation:
AB=√AD²+BD²
AB=√5²+4²
AB=√25+16
AB=√41
cos∡ABC=AB/BC=BD/AB
AB/BC=BD/AB
AB²=BC*BD
(√41)²=BC*4
41/4=BC
BC=10.25
