Question

(11-2 с)(5с-6)=80-10(с-3)^2

Answer 1

$\displaystyle (11-2c)(5c-6)=80-10(c-3)^2\\55c-66-10c^2+12c=80-10(c^2-6c+9)\\55c-66-10c^2+12c=80-10c^2+60c-90\\55c-66+12c=80+60c-90\\67c-66=-10+60c\\67c-60c=-10+66\\7c=56\\c=8$