If a canoe travels with a speed of a mph for 3 hours, and then with a speed of b mph for the rest of the journey, how long does it take to travel c miles

For time-speed-distance problems, you use the relationship
.. speed = distance/time
in any of its various forms.

The distance traveled at A mph is
.. distance = speed * time
.. distance = A*3

The time required for the rest of the journey is
.. time = distance / speed
.. time = (C -3A)/B

The the total time required to travel C miles is the 3 hours for the first part plus the time for the rest of the journey.
.. total journey time = 3 + (C -3A)B . . . . . hours

The length of what is left would be 1 1/2 feet long.