[tex]x=7\\\\\text{because}\\\\a^{-n}=\dfrac{1}{a^n};\ a\neq0\\\\(-j)^{-7}=\dfrac{1}{(-j)^7}[/tex]
[tex]\text{Minus in a power means inverse.}[/tex]
[tex]a^{-1}=\dfrac{1}{a}\\\\(-j)^{-7}=(-j)^{7\cdot(-1)}=\left[(-j)^7\right]^{-1}=\dfrac{1}{(-j)^7}\\\\\text{Used:}\\\\(a^n)^m=a^{n\cdot m}[/tex]
