Х³-3х²-ху²+3у²=х²(х-3)+у²(3-х)=х²(х-3)-у²(х-3)=(х²-у²)(х-3)=(х-у)(х+у)(х-3)
x- cкорость течения
44/(20+x) + 54/(20-x) = 7-2
44(20-x) + 54(20+x)=5(400-x^2)
880 - 44x + 1080 + 54x = 2000 - 5x^2
1960 + 10x = 2000 - 5x^2
5x^2 + 10x - 40 = 0
x^2 + 2x - 8 = 0
x1 = 2 x2=-4
Ответ: 2 км/ч