100-49х2у
(5+х)2-9
(2а+7)2-(3а-5)2

100-49х^2у^2 =10²-(7xy)²=(10-7xy)(10+7xy)

(5+х)^2-9 =(5+x+3)(5+x-3)=(8+x)(2+x)

(2а+7)^2-(3а-5)^2=(2a+7+3a-5)(2a+7-3a+5)=(5a+2)(12-a)