Explanation:
(3x-1)(x+1)=3(3x-4+x²)
open the brackets
=3x²+3x-x-1=9x-12+3x²
=3x²+2x-1=9x-12+3x²
collecting like terms
=3x²-3x²+2x-9x=-12+1
=-7x=-11
=x=11/7
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