$36\,y^2 \,- 100 = $   ?

$(36y+100)(36y-100)$ $(6y+10)^2$ $(6y+10)(6y-10)$ $(6y+100)(6y-100)$

$16\,y^2 \,- 144\,x^2 = $   ?

$(4y+12x)(4y-12x)$ $(4y+12x)^2$ $(4y-12x)^2$ $(16y+12x)(16y-12x)$

$4\,y^2 \,- 81 = $   ?

$(2y-9)^2$ $(2y+9)(2y-9)$ $(2\,y^2+9)(2\,y^2-9)$ $(4y+9)(4y-9)$

$81\,y^2 \,- 49 = $   ?

$(9y+49)(9y-49)$ $(9\,y^2+7)(9\,y^2-7)$ $(9y+7)(9y-7)$ $(81y+7)(81y-7)$