1. **State the problem:** We need to add two numbers in standard form: $3.9 \times 10^9$ and $6.4 \times 10^7$. 2. **Recall the rule for addition in standard form:** To add numbers in standard form, they must have the same power of 10. If not, convert one number so both have the same exponent. 3. **Rewrite the numbers with the same power of 10:** $6.4 \times 10^7 = 0.064 \times 10^9$ (because $6.4 \times 10^7 = 6.4 \times 10^{7} \times \frac{10^2}{10^2} = 0.064 \times 10^9$). 4. **Add the coefficients:** $3.9 + 0.064 = 3.964$ 5. **Write the sum in standard form:** $3.964 \times 10^9$ 6. **Check if the coefficient is between 1 and 10:** $3.964$ is between 1 and 10, so the number is already in standard form. **Final answer:** $$3.964 \times 10^9$$