If i row my canoe at 5 mph and i travel upstream at 3 mph, how fast will i be traveling downstream if i am rowing at the same rate ?

Is the same as the one you just did.

keep in mind that, going against the current, the current's speed erodes speed from your regular speed, whilst if you're going with the current, the current's speed adds to it.

now, in this case, you row 5mph, going upstream you're only doing 3mph, whatever happened to the other 2mph?  well, the current speed eroded them, meaning the speed of the river is 2mph.

now, going downstream with the current, your regular speed is 5mph, and the current is 2mph, since the current adds to yours, then you're going 5 + 2, mph.

Subtracting is the opposite of adding