First, we need to make the exponent of 10 the same for both.
So let's transform 6.5500 x 10^5 into some number x 10^3.
For this, we need to move the dot to the right, some places where it gives the number 3. In this case, 2 places.
655.00 x 10^3
now we can sum the numbers
8.6500 x 10^3 + 655.00 x 10^3 = 663.65 x 10^3
now we need to transform this number into scientific notation. For this, must have only one number before the dot(on the left side of the dot). We will move the dot to the left, 2 places:
6.6365 x 10^5
Answer: 6.6365 x 10^5