How would you find the values of $\displaystyle x$ for these logarithms:

a.$\displaystyle log_x \frac {1}{5} = -\frac {1}{4}$

b.$\displaystyle log_x 81 = \frac {3}{4}$

I put both of them in exponential form:

a.$\displaystyle x^\frac {-1}{4} = \frac {1}{5}$

b.$\displaystyle x^\frac {4}{3} = 81$

Then I got stuck in trying to figure out what the value of "x" equals. I knw how to find it using a calculator but I can't find the value without using a calculator.

And one more thing. How do you find the inverse fuction of $\displaystyle f(x)=2log_2 x$?