BC = x
AB = BC - 6
AC = BC + (BC - 6) - 13 = 2BC - 19
AB + AC + BC = 78
substitute:
(BC - 6) + (2BC - 19) + BC = 78
4BC - 25 = 78 |+25
4BC = 103 |:4
BC = 25.75
Answer: 25.75 mm
