To add the numbers \( 7.3 \times 10^7 \) and \( 1.8 \times 10^7 \), you can first combine their coefficients, since they have the same exponent.
\[ 7.3 + 1.8 = 9.1 \]
Now, keeping the common exponent of \( 10^7 \), you get:
\[ 9.1 \times 10^7 \]
So, the sum of \( 7.3 \times 10^7 + 1.8 \times 10^7 \) is:
\[ 9.1 \times 10^7 \]