Writing both terms in scientific notation, we have:
7166.0 = 7.1660*10³
So, this term has 4 significant figures (.1660)
2.4*10^23 is already written in scientific notation. So it has 1 significant figure (.4)
Now, when summing or subtracting two quantities, the number of significant figures of the result equals the smaller number of significant figures. So, in this case, the result has 1 significant figure.
Then, proceeding with the calculation, we have:
7166.0 - 2.4*10^23 = 7.1660*10³ - 2.4*10^23 = (7.1660 - 2.4*10^20) * 10³
= (7.1660 - 240,000,000,000,000,000,000.0) * 10³
= -239,999,999,999,999,999,992.8340
Finally, we need to round the first significant figure, so there will be only one significant figure in the result:
-239,999,999,999,999,999,992.8