Let u = the canoe speed in still water (the speed relative to water), in mph.
    v = the speed of the current.

When canoe travels downstream, its speed relative to the bank of the river is the sum u + v, and it is equal to

u + v =      (speed = ).

When canoe travels upstream, its speed relative to the bank of the river is the difference u - v, and it is equal to

u - v =      (again, speed = ).

Rewrite these equations with calculated right sides:

u + v = 5,      (1)
u - v = 3.      (2)

Now add equations (1) and (2) (both sides). You will get

2u = 5 + 3 = 8.

Hence, u =  = 4 mph. It is the canoe speed in still water.

Next,  from (1)  v = 5 - u = 5 - 4 = 1 mph. It is the current speed.