In △ABC, two distinct lines each parallel to BC intersect AB and AC. The line nearer to BC meets AB at D and AC at E, while the line nearer to A meets AB at F and AC at G (with F between A and D). Given AD=6cm, DB=12cm, AF=4cm and AG=5cm, find the length of GC.
(a) 15.5cm
(b) 19.5cm
(c) 17.5cm
(d) 21.5cm