Write both number with the same power of 10, factor out that power of 10 (since it's common to both terms), then simplify to standard scientific notation (that is, a.bcd... × 10⁻ⁿ):
(8.28 × 10⁻⁵) - (4.1 × 10⁻⁷) = (828 × 10⁻⁷) - (4.1 × 10⁻⁷)
… = (828 - 4.1) × 10⁻⁷
… = 823.9 × 10⁻⁷
… = 8.239 × 10⁻⁵
