In order to now how to add or subtract number in sicentific notation, the most important thing is that the number have the same exponent in the factor 10.
For example, you can add or subtract the following numbers in scientific notation:
4.5x10⁵ + 6.1x10⁵
you can add the prvious numbers because they have the same exponent in the factor 10 (in this case the exponent is 5).
After you notified the exponents are equal, you can add the numbers. This cosists simply in adding the coefficients, which in this case are 4.5 and 6.1.
You add these number and to the result you put the factor 10 with the same exponent, that is, you put 10⁵.
Summarizing, if you want to add two sicentific numbers as 4.5x10⁵ + 6.1x10⁵, you add the coefficients 4.5 and 6.1 and put the factor 10⁵.
The sum of 4.5 + 6.1 = 10.6, then you have:
4.5x10⁵ + 6.1x10⁵ = 10.6x10⁵
In the case of subtraction, it is exactly the same case as addition. Just calculate the difference between coefficients and put the same factor 10 with the same exponent.
For example:
3.2x10⁴ - 2.1x10⁴ = 5.3x10⁴