3{10log5.2}

log(x) means "the power to which 10 must be raised to get the answer x".

Therefore

10log5.2 means "10 raised to the power to which 10 must be raised to
get the answer 5.2.

Now if you'll think about what that says you'll have it.

If you raise 10 to the power that you must raise 10 to to get 5.2 you must get
5.2.

So 10log5.2 is just 5.2 and

3(5.2) is 15.6

Edwin