Решите уравнение, прошу: (7 х+1) (х-3) + 20 (х-1) (х+1) = 3 (3 х-2) ²+13

(a-b) ²=a²-2ab+b²

a²-b² = (a-b) (a+b)

(7 х+1) (х-3) + 20 (х-1) (х+1) = 3 (3 х-2) ²+13

(7 х+1) (х-3) = 7x²+x-21x-3=7x²-20x-3

(х-1) (х+1) = x²-1²=x²-1

(3 х-2) ²=3²x²-2*3x*2+2²=9x²-12x+4

(7 х+1) (х-3) + 20 (х-1) (х+1) = 3 (3 х-2) ²+13

(7x²-20x-3) + 20 (x²-1) = 3 (9x²-12x+4) + 13

7x²-20x-3+20x²-20=27x²-36x+12+13

7x²+20x²-27x²-20x+36x=12+13+3+20

16x=48

x=48:16

x=3