Just a Few Calculus Assignment Questions

Alright, I'm back and looking for somemore help :)

1) I'm having some trouble finding the X intercept using logarithms for y=2(0.5)^X-6. Here are my steps so far:

log0= xlog(2(0.5))-6

log0+6/log(2(0.5))=x

This won't let you get an answer, I assume this is because log(1) is 0 because that is what my calculator is telling me. According to the graph I made with table of values, the x intercept should be arround -1.75.

Also, that question is asking for the equation of the asymptotes. Can anyone fill me into what that would be?

2) Express the function y=5^x as an equivilent function with base 3.

3) Suppose money can be invested at 3.75% compounded semi-annually. Determine how long it takes for a principle to triple in value. (I have an idea on how to do this up to a certain point but then I get stuck.)

4) The equation D= 10logI models the decible level of a sound whose intensity is I watts per sq. meter W/m2. The decible levels of a subway and normal conversation are 115dB an 60db respectivly. How many times as intense as normal conversation is the noise of a subway?

If anyone could help with any of these I'd be very greatful. This is a wonderful resource for the struggling calc. student.