`a)|x-3|=7-(-2)`
`→|x-3|=9`
`→` \(\left[ \begin{array}{l}x-3=9\\x-3=-9\end{array} \right.\)
`→` \(\left[ \begin{array}{l}x=12\\x=-6\end{array} \right.\)
Vậy `x∈{12;-6}`
`b)4-(7-x)=x-(13-4)`
`→4-7+x=x-13+4`
`→x-x=-13+4-4+7`
`→0=-6` (không thoả mãn)
Vậy `x∈∅`