Respuesta :

-2x^2+50x+40=0
-2*(x^2-25x-20)=0
x^2-25x-20=0

complete square:
x^2-25x-20+156.25^2=0+156.25
x^2-25x+156.25^2=20+156.25
(x-12.5)^2=176.25
x-12.5=+/-sqrt(176.5)
x=12.5+/-sqrt(176.5)