(5а-5б)х(3а+4б)=(5а+3а)(-2х+4х)=8а х 2х

(5a-5b)(3a+4b)=(5a+3a)(-2x+4x)=8a*2x

15a^2+20ab-15ab-20b^2=8a*2x=8a*2x

15a^2+5ab-20b^2=16ax=16ax

15a^2+5ab-20b^2=16ax-16ax

15a^2+5ab-20b^2= 5(3a^2+ab-4b^2)