Estimations like this usually involve rounding to easy numbers to multiply. You could do this in a number of ways depending on your ability to keep track of factors in your head, but the easiest way (and what I assume your teacher is looking for) is to round to the nearest, largest-power-of-ten, which in this case is the hundreds. 786 is closest to 800, and 217 is closest to 200, and 200*800 is 160,000. No calculator required is the key. You can also be slightly more sure that this is <em>reasonable</em> because you rounded up for one and down for the other, approximately by the same amount, too.
