M Is The Midpoint Of Segment AB, AM=2x+4 And AB=12x+4. What Is The Value Of MB? Show Your Work For Credit.

If M is the midpoint of AB, then AM = MB. AM is given, so

MB = 2x+4

Perhaps you are interested in a numerical solution.
AB = AM + MB (the length of the segment is the sum of the lengths of its halves)
12x+4 = 2(2x+4) = 4x+8 (each half is 2x+4; use the distributive property)
8x + 4 = 8 (subtract 4x from both sides)
8x = 4 (subtract 4 from both sides)
x = 4/8 = 1/2 (divide both sides by 8)

MB = 2(1/2) + 4 = 5