Answer:
x = 10
Explanation:
You want to find the value of x when B is the midpoint of AC, and AB = 3x-8 and BC = x+12.
Setup
A midpoint divides a segment into two congruent segments:
AB = BC
3x -8 = x +12
Solution
Adding 8-x to both sides, we get
2x = 20
x = 10 . . . . . . . divide by the coefficient of x
The value of x is 10.
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Additional comment
The lengths of AB and BC are 10+12 = 22, which makes the length of AC = 44.