$49 \,- 16\,y^2 = $   ?

$(49+16y)(49-16y)$ $(7+4y)(7-4y)$ $(7+4\,y^2)(7-4\,y^2)$ $(7-4y)^2$

$100\,y^2 \,- \,x^2 = $   ?

$(10y+x)(10y-x)$ $(10y-x)^2$ $(10y+x)^2$ $(100y+x)(100y-x)$

$36 \,- 9\,y^2 = $   ?

$(6-3y)^2$ $(6+3y)^2$ $(6+3y)(6-3y)$ $(36+9y)(36-9y)$

$36\,x^2 \,- 49 = $   ?

$(6x-7)^2$ $(6x+7)^2$ $(6x+49)(6x-49)$ $(6x+7)(6x-7)$