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HistoryNerd15

Because B is the midpoint of AC and AB = 2x + 4, BC must also be 2x + 4. So you add 2x + 4 to 2x + 4 to get 4x + 8. Then you set that to be equal to 40. Then you subtract 8 from both sides to get 4x = 32 and divide by 4 so you end up with x = 8.