To find the sum of the populations of the two cities given in scientific notation:
- The population of the first city is \( 3.45 \times 10^6 \).
- The population of the second city is \( 1.3 \times 10^5 \).
Before we can add these two numbers, they should be expressed with the same exponent. The larger exponent is \( 10^6 \), so we'll convert the population of the second city to the same exponent:
\[ 1.3 \times 10^5 = 1.3 \times 10^5 \times \frac{10^1}{10^1} = 1.3 \times 10^6 \div 10 = 0.13 \times 10^6 \]
Now we can add the two populations in scientific notation:
\[ 3.45 \times 10^6 + 0.13 \times 10^6 = (3.45 + 0.13) \times 10^6 = 3.58 \times 10^6 \]
Thus, the sum of the populations of the two cities in scientific notation is:
\[ \boxed{3.58 \times 10^6} \]