hmmm let's find the distance between B and C first off.

hmmm if BC is that much, then AB + AC = 7, since AB + AC + BC = 12 which is the perimeter, Check the picture below.
Median = 15.5
Please take a look at the image attached for an explanation.
Cross multiply
18 / X = 2.4 / 28
so
18 * 28 = X (2.4)
and that becomes
504 = X * 2.4
then divide by 2.4
X = 210