Add or subtract as indicated.
a. 5.80 x 10^9 s +3.20 x 10^8 s
b. 4.87 x 10^-6 m - 1.93 x 10^-6 m

In order to add or subtract numbers expressed in scientific notation, it is important that the powers of 10 in the numbers should have the same index or else you cannot readily add or ubtract these numbers. We do as follows:
a. 5.80 x 10^9 s +3.20 x 10^8 s
We need to adjust one of the numbers since they do not have the same indices. We move one place to the left of the second number making it .320 x 10^9 s. Then, we can add the numbers now:

5.80 x 10^9 s +.320 x 10^9 s = 6.12 x 10^9 s

b. 4.87 x 10^-6 m - 1.93 x 10^-6 m

We can subtract the numbers directly since they have the same indices.

4.87 x 10^-6 m - 1.93 x 10^-6 m = 2.94 x 10^-6 m

It should be d if not then c