The key step is to rewrite the exponential notations so that the exponent is the same. Then you can deal with the addition just as you would adding two decimals. The best way is to rewrite the number with the smaller exponent.
6.30 x 10^11 = 0.0630 x 10^13
Now you are adding 6.28 x 10^13 and 0.0630 x 10^13, and your answer is (6.28 + 0.0630) x 10^13. You can do the addition in the parentheses using the usual rules for sig dig when you add decimals. If necessary, afterward, you can rewrite the exponent again so that the mantissa is between 1 and 10.
