It is also more direct to calculate Voyager's speed as a fraction of the speed of light and then divide that into the 21 light years.

  Voyager goes 35,000 miles per hour (given)
           ... 10 miles per second
  Light goes 187,000 miles per second (given)
  Voyager goes 1/2000 the speed of light
  Light takes 21 years to get there (given)
  Voyager takes 42,000 years to get there.

Hopefully I didn't drop too many zeros or use the wrong reciprocal too many (non-canceling) times.

But Voyager was not intended to go fast by interstellar speeds. It just needed to get to all of its planets before its electronics died. Any speed beyond that was a waste and could have been traded off against more data gathering equipment.

When it comes time to send the nanobots with the directions to fabricate exploration gear and giant space arrays to transmit back the results to us, I'm sure we can make them go faster.