Answer
$(5x+3)(5x-3)$
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 by deducing the area of the white square from the big one: $A=5x\cdot 5x-3\cdot3=(5x)^2-3^2=(5x+3)(5x-3)$