To add or subtract numbers written in scientific notation, the powers of ten must have identical exponents. If they do, simply add/subtract the leading coefficients and keep the exponent the same.
If the sum is not less than ten/greater than one, the decimal must be moved and exponent adjusted properly.
To multiply or divide numbers in scientific notation, rearrange the factors and group the coefficients and powers of 10. Multiply/divide the coefficients, then multiply the powers of 10 (by adding/subtracting the exponents). Then write the number in scientific notation.