If
#color(white)("XXX")2x^3-8x=-8#
then
#color(white)("XXX")x^2-4x=-4# (after dividing by #2#)
#color(white)("XXX")x^2-4x+4=0# (after adding #4# to both sides)
#color(white)("XXX")(x-2)^2=0# (factoring)
#color(white)("XXX")(x-2)=0# (since if #a*b=0# then #a=0# or #b=0#)
#color(white)("XXX")x=2# (after adding #2# to both sides)