a^2 -9ac -36c^2 = a^2 -2*a *4.5c +20.25c^2 -20.25c^2 -36c^2 = a^2 -9ac -+20.25c^2 -56.25c^2
(a - 4.5c)^2 -56.25c^2 = (a -4.5c -7.5c)(a -4.5c +7.5c) =(a -12c)(a +3c)
