Question 605780
{{{10^(-3x)=7}}}

A general rule is whenever you have a variable in the exponent that you are trying to solve for, take the log (or ln) of both sides so you can use the log rules to bring down the exponent.  this leaves:
{{{log(10^(-3x))=log(7)}}}
{{{(-3x)*log(10)=log(7)}}}
And now isolate your variable:
{{{x=(log(7))/(-3*log(10))}}}
Now, we know that since 10^1=10, log10=1
So we have:
{{{x=(log(7))/(-3)}}}
This is just a number, you can find a decimal number using your calculator, or you can leave it as we have above, either is correct.  
Hopefully this helps:)