Let b represent the speed of the boat in still water.
The boat's speed downstream will be b + 3.
The boat's speed upstream will be b - 3.
Distance = rate x time
Upstream: 36 = (b - 3)(t) where t represents the time.
Downstream: 62 = (b + 3)(t) same time upsteam as downstream
Solving the top equation for t: 36/(b - 3) = t
Solving the second equation: 62/(b + 3) = t
Since they both equal t, the equal each other: 36/(b - 3) = 62/(b + 3)
Cross-multiplying: 36(b + 3) = 62(b - 3)
Simplifying: 36b + 108 = 62b - 186
Simpligying: 108 = 26b - 186
Simplifying: 294 = 26b
Dividing: b = 11.3 mph (approximately)