$(x-3)(x+3)-(x-3)(x+5)=0 \Rightarrow (x-3)[x+3-(x+5)]=0$
$(x-3)(x+3)=(x-3)(x+5) \\ (x-3)(x+3-x-5)=0 \\ -2(x-3)=0 \\ x-3=0 \\ x=3$