The way the problem is worded, I'm assuming the boat traveled 126 miles each way.
Velocity against the current 126 / 84 = 1.5 miles per hour
Velocity with the current 126 / 12 = 10.5 mph
"cs" means current speed
"bvs" means boat velocity in still water
bvs -cs = 1.5 mph
bvs +cs = 10.5 mph
Adding both equations
2 bvs = 12 mph
boat velocity in still water = 6 mph
and since
bvs +cs = 10.5 mph
then
6 + cs = 10.5
current speed = 4.5 mph