39. $$4^{x-2} = (\frac{1}{2})^{1-x}$$

$$4^{x-2} = \left(\frac{1}{2}\right)^{1-x}$$ $$(2^2)^{x-2} = (2^{-1})^{1-x}$$ $$2^{2x-4} = 2^{-1+x}$$ $$2x-4 = -1+x$$ $$x = 3$$ Ответ: $$\boxed{x=3}$$