When you have a term raised to a negative exponent, you take the reciprical of the term.

(2b^4)^-1 = 1/(2b^4)

(x^2y^-1)^-1 = 1/(x^2y^-1) = y/x^2 <-- notice that the y had a negative exponent and so could be brought back up the numerator

x^2/(2x^3) <-- in this case, we need to subtract the exponents since the "x"s are the same base. We get

1/2*x^(2 - 3) = 1/2*x^-1 = 1/(2x)

If that's not clear, say so and I'll try to explain it better.