13. (a-b)^2=a^2-2ab+b^2 B is the answer 14. B is the answer ( x+9)(x-3)= x^2+6x-27 15. B is the answer (x+6)(x-5)=x^2+x-30 16. D is the answer (x-6y)(x-4y)=x^2-10xy+24y^2 17. C is the answer (2x+5)(3x-4)=6x^2+7x-20 18. B is the answer (5x+7)(x-2)=5x^2-3x-14