Finding H3O+ concentration with pH?

In order to obtain the proper numbers of sig figs when using logs and exponentials the rule is that when you raise something to a power or take the log of some number, you need to report the answer with as many numbers after the decimal point as there are sig figs. So, in your situation, when you do 10^(-7.4) you, the answer is 3.98 x 10^-8 because 7.4 is two sig figs, so you report ur answer to 2 decimal places..