what is log(7^(2x+3)) equal to? how do you simplify this?

You are not given the base so I think you should just leave it as  (2x+3)log7

By convention, no base in the log function is assumed to be base 10

Well the question says equal to.   If you get a value of log7 base 10 on your calculator is it an approximation.

THAT's dipping pretty deep, Melody !      Virtually all answers using pi are then approximations and not 'equal to' unless expressed in terms of pi....a large majority of answers on this forum are rounded and thus approximations despite the = sign   etc.....

I only give approximations when the question asks for, or implies the need for or the benefit of, an approximation. 

If I have ever done it at other times then my answer is not as good as it could have been.

If it is a practical question then an approximate answer is most likely best.

If it is a pure mathematics question, such as this one, then the precise answer is the best one.