I believe the givens should be written in this manner,
(4.2 x 10⁵)(4.0 x 10⁻⁹)
To answer, we first multiply the bases 4.2 and 4 which will give us,
16.8
Then, we multiply the multipliers by adding up the exponents of 10. This will give us the answer of,
16.8 x 10⁻⁴
Then, the final answer would be 1.68 x 10⁻³.
