x=-\frac{1}{6}
(2x-3) (2x-3) - (2x+3) (2x+3)=(2x-3)^2-(2x+3)^2=
[2*(-\frac{1}{6})-3]^2-[2*(-\frac{1}{6})+3]^2=(-\frac{1}{3}-3)^2-(-\frac{1}{3}+3)^2=
(-3\frac{1}{3})^2-(2\frac{2}{3})^2=(-\frac{10}{3})^2-(\frac{8}{3})^2=\frac{100}{9}-\frac{64}{9}=\frac{36}{9}=4