`a, x^2 - x - 6 = x- 3`
`(x-3)(x+2) = x-3`
`(x-3)(x+2-1) = 0`
`<=> (x-3)(x+1) = 0`
`<=>` \(\left[{}\begin{matrix}x=3\\x=-1\end{matrix}\right.\)
`b, x^2 - x = 3x - 5`
`x^2 - 4x - 5= 0`
`(x-5)(x+1) = 0`
`<=>` \(\left[{}\begin{matrix}x=5\\x=-1\end{matrix}\right.\)