Here's how to round 0.000 763 21 cg to three significant figures:

1. Identify the Significant Digits:

* Non-zero digits are always significant. In this number, the significant digits are 7, 6, and 3.

2. Determine the Rounding Digit:

* The third significant digit is 3. We look at the digit to its right (2) to determine whether to round up or down.

3. Round Up or Down:

* Since 2 is less than 5, we round down.

4. The Result:

* Rounded to three significant figures, 0.000 763 21 cg becomes 0.000 763 cg.

Important Note: Leading zeros before the first non-zero digit are not significant.