Verified answer

(7х+1)(х-3)+20(х-1)(х+1)=3(3х-2)²+13

7x²-21x+x-3+20(x²-1)=3(9x²-12x+4)+13

7x²-20x-3+20x²-20=27x²-36x+12+13

27x²-20x-23=27x²-36x+25

27x²-27x²-20x+36x=23+25

16x=48

x=48/16=3

Ответ: 3.