You can only add or subtract numbers in scientific notation if they have
the same power of 10. If they're different, then you have to change one
to match the other one.
Here are both ways to do your example:
3.72x 10^9 = 37.2x10^8
(37.2 x 10^8)+(5.46 x 10^8) = (37.2+5.46) x 10^8 = 42.66x10^8 = 4.266x10^9
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5.46 x 10^8 = 0.546 x 10^9
3.72 x 10^9 + 0.546 x 10^9 = (3.72 + 0.546) x 10^9 = 4.266 x 10^9
