`D = (0; +infty)`
`2log_{7} x + log_{49} x^{-1} = 0`
`-> log_{7} x^{2} + log_{7^{2}} (1/(x)) = 0`
`-> log_{7} x^2 + 1/(2)log_{7} (1/(x)) = 0`
`-> log_{7} x^2 + `$log_{7} (\dfrac{1}{x})^{\dfrac{1}{2}}$ ` = 0`
`-> log_{7} x^2 + log_{7} ((1)/(\sqrt{x})) = 0`
`-> log_{7} ((x^2)/(\sqrt{x})) = 0`
`-> (x^2)/(\sqrt{x}) = 7^0 = 1`
`-> x^2 = sqrt{x}`
`-> x^4 - x = 0`
`-> x(x^3 - 1) = 0`
`->` \(\left[ \begin{array}{l}x=0 (l)\\x= 1\end{array} \right.\)
`-> S = {1}`
