16x^2 - 81y^2
x^2-25
49c^2 - 9
a^4-4b^2
(4x-9y)*(4x+9y)
16х^2+36yx-36yx-81y^2=16x^2-81y^2
(x-5)*(x+5)
x^2+5x-5x-25=x^2-25
(7c+3)*(7c-3)
49c^2-21c+21c-9=49c^2-9
(a^2-2b)*(a^2+2b)
a^4+2ba^2-2ba^2-4b^2=a^4-4b^2
