$36\,x^2 \,- 36\,y^2 = $   ?

$(6x-6y)^2$ $(6x+6y)(6x-6y)$ $(36x+6y)(36x-6y)$ $(6\,x^2+6\,y^2)(6\,x^2-6\,y^2)$

$81\,y^2 \,- 16\,x^2 = $   ?

$(9y-4x)^2$ $(81y+16x)(81y-16x)$ $(81y+4x)(81y-4x)$ $(9y+4x)(9y-4x)$

$144\,y^2 \,- 36 = $   ?

$(12y+6)(12y-6)$ $(144y+36)(144y-36)$ $(144y+6)(144y-6)$ $(12y+6)^2$

$49 \,- 4\,x^2 = $   ?

$(7-2x)^2$ $(7+2x)(7-2x)$ $(7+2\,x^2)(7-2\,x^2)$ $(49+2x)(49-2x)$