Answer
$(5x-2-9y)(5x-2+9y)$
Work Step by Step
The product of a binomial sum and a binomial difference is: $(A+B)(A-B)=A^2-B^2$.
The square of a binomial sum is: $(A+B)^2=A^2+2AB+B^2$.
The square of a binomial difference is: $(A-B)^2=A^2-2AB+B^2$.
Hence here: $25x^2-20x+4-81y^2=\\=(5x)^2-2\cdot2\cdot5x+2^2-(9y)^2=\\=(5x-2)^2-(9y)^2\\=(5x-2-9y)(5x-2+9y)$