Given that log 3 base ten equal A and log Five base ten equal B express log 75 base ten in terms of A and B
A = log3
B = log5
log75 = log(3*5^2) = log3 + 2log5 = A+2B
as long as all the bases are the same, the calculations work out the same.
Usually, in the beginning, or in engineering, log with no base means log base 10; common log