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Answer:
BC = 13.4
Step-by-step explanation:
its a law of cosines S-A-S
a² = b² + c² - 2bc cosA
a² = 12.6² + 4.6² - ( 2 * 12.6 * 4.6 * cos 90 )
a² = 179.92
a = sqrt (179.92)
a = 13.4