Explanation:
If M is the midpoint of AB then,
AM = BB so we can write the following equation:
4x + 13 = 3x + 17
- transfer like terms to the same side of the equation
4x - 3x = 17 - 13
x = 4
Now on to the length of BM, we can replace x with 4 to find it.
3*4 + 17 = 29