Question

$(x-5)^{2} = -20x$

Answer 1

$(x-5)^{2} =-20x \ x^{2} -10x+25=-20x \ x^{2} +10x+25=0$
по теореме Виета
$left { {{x1+x2=-10} atop {x1*x2=25}} right.$
$x1=-5 \ x2=-5$
Ответ: x=-5