d=rt
c=speed of current
r=speed in still water
t=time
d=72
72=3(r+c)
72=4(r-c)
expand
divide top equation by 3 and bottom by 4
24=r+c
18=r-c
add them equations
42=2r+0c
42=2r
divide both sides by 2
21=r
sub back
18=r-c
18=21-c
minus 21 both sides
-3=-c
3=c
speed of current is 3
speed of ship is 21