Answer:
x = 10
Explanation:
The angle bisector divides the triangle into proportional parts, so ...
(2x+1)/12 = (x+4)/8
8(2x+1) = 12(x+4) . . . . cross multiply
16x +8 = 12x +48 . . . . eliminate parentheses
4x = 40 . . . . . . . . . . . . subtract 12x+8
x = 10 . . . . . . . . . . . . . divide by 4
_____
Comment on this answer
The answer x=10 satisfies the requirement for proportional line segments, but it makes segment BC be 35 units long. It cannot be longer than BA+AC = 20 in order for the segments to connect to form a triangle. The geometry shown in this problem statement is impossible.