it is given that
BD = median
also,
AC = 54x - 5
and
DC = 12x + 20
we know that the median of the triangle divides the length AC in equal two parts,
so,
AC = 2 x DC
put the values,
54x - 5 = 2(12x + 20)
54x - 5 = 24x + 40
54x - 24x = 40+ 5
30x = 45
x = 45/30
x = 3/2
x = 1.5
put x = 3/2 in equation AC = 54x - 5
AC = 54 x (3/2) - 5
= 27x 3 - 5
= 81 - 5
= 76
thus, the answer is
x = 3/2 = 1.5
AC = 76