text: This is pretty simple \n\n4x^2 = (2x)^2\n81 = 9^2\nis -36x = -2(2x)9 yes\n\nso we get((2x)^2-2(2x)9+9^2) = (2x-9)^2\n\nAnother way\ntry to sqaure it by putrting 81 = a + b so that \n\n4x^2-36x+a is a perfect square\n(2x)^2-2(2x)9 + +9^2 is a perfect square\n\nso a =81 and b =0 \nwe get(2x-9)^2\n\n1st method works when we know the pattern and second method for all cases. whole square falls into it