Final answer:
The completed format string to output 'Answer: 0.99' using the number 0.987654 is 'Answer: {:.2f}'.format(0.987654), which specifies two decimal places of precision.
Step-by-step explanation:
The format string should specify the precision of the floating-point number that you want to include in the output. To match the output 'Answer: 0.99' when formatting the number 0.987654, you'll need to round the number to two decimal places. You can do this by including :.2f within the braces of the format string, which indicates a fixed-point number with two digits after the decimal point.
Here is the completed format string:
'Answer: {:.2f}'.format(0.987654)
When this format string is used, the number 0.987654 will be formatted to two decimal places, producing the desired output.