Find the x-intercept of y-7=3^(x+4).
My answer: ( (log base 3 of -7) minus 4)
you want y=0, so
3^(x+4) = -7
Now, you know that 3^n > 0 for any n, so there is no solution.
Similarly, your "solution" is invalid because log(n) is not defined for n <= 0.