Austin is in his pedal boat. It takes him 1.5 hours to travel 12 miles downstream and 6 hours to return. What is the rate of the current?

Please show me how to solve it as well, I really have no idea. Thanks.

if boat speed = b, and river speed = r, then since speed = distance/time,

b+r = 12/1.5 = 8 (downstream)
b-r = 12/6 = 2 (upstream)

b=5,r=3