+130515 I assume you mean this:
log3 7 ...... We can use the "Change of Base" rule to evaluate this....it says that the "power" - let's call it "x" - needed to raise 3 to 7 is given by:
x = log(7)/log(3) ≈ 1.7712437491614222
Check for yourself that 31.7712437491614222 ≈ 7
