Whats 7x10 to the -8 power + 2 x 10 to the -8 power

9x10 to the -8th power. 

Since the numbers are both x10^-8, you can simply add 2 and 7. If you would rather look at it as:
0.00000007 + 0.00000002 = 0.00000009 = 10x10^-8
then you will get the same answer.