To multiply numbers in scientific notation, first multiply the numbers that do not have a base of 10 and then add the exponents of the bases of 10. Essentially:
In this case, we see that the numbers that do not have bases of 10 are 2.3 and 3.1. Multiplying these numbers gives us:
The exponents on the bases of 10 are -5 and 1. Adding these exponents gives us -4, which we can add to our final base of 10.
Thus, our answer is: