Answer:
x = 18
Explanation:
I the given triangle, it appears that M and N are the midpoints of the segments BG and BD respectively. If it so, then let us solve it.
By mid segment theorem:
2MN = GD
2(6x - 51) = 114
12x - 102 = 114
12x = 114 + 102
12x = 216
x = 216/12
x = 18