To add \(7.3 \times 10^7\) and \(1.8 \times 10^7\), you can combine the coefficients since the powers of 10 are the same.
\[ 7.3 + 1.8 = 9.1 \]
So,
\[ 7.3 \times 10^7 + 1.8 \times 10^7 = (7.3 + 1.8) \times 10^7 = 9.1 \times 10^7 \]
The correct sum is:
9.1×10^7 (9.1 times 10 superscript 7 baseline).