The given expression is
(5.25x10^-3)(2.5x10^-4)
We can re-write it as shown below
5.25 * 2.5 * 10^- 3 * 10^- 4
Since the exponents has a common base of 10, we can apply the law of exponents which is expressed as
a^x * a^y = a^(x + y)
It becomes
5.25 * 2.5 * 10^(- 3 + - 4)
= 13.125 * 10^(- 3 - 4)
= 13.125 * 10^- 7