2,138,056,873,500 to 3 significant places

Just count three of the digits from the left and inspect the fourth.

The third digit is 3 and the fourth is 8, and it is more than five, use we take one from the 8 and add up to the 3, which becomes 4, If the fourth digit was not more than 5, then we would not have added to the third digit, we would have left it that way and replaced with zeros.

And we replace the other digits with zeros.

2 138 056 873 500 = 2 140 000 000 000 to 3 significant places.

Cheers.

2,140,000,000,000 is your answer, as significant figures are figures that are not 0.

The only exception to this rule is that if you were to take 280743084 and round it to 4 sig fig, you would get the answer, 280700000, as you include 0s when there are other numbers next to them.