Answer:
If m is the midpoint of ab it means am is equal to mb
am = 3x + 8 and mb = 6x-4
and ab = am + mb
first find the value of x by using am = mb
3x + 8 = 6x - 4
8 + 4 = 6x - 3x
12 = 3x
x = 4
ab = 3 (4) + 8
= 12 + 8 = 20 = mb
ab = am + mb
= 20 + 20
40
so, the value of ab is 40