$144\,y^2 \,- 81\,x^2 = $   ?

$(144y+81x)(144y-81x)$ $(12y+9x)^2$ $(12\,y^2+9\,x^2)(12\,y^2-9\,x^2)$ $(12y+9x)(12y-9x)$

$49\,x^2 \,- 36\,y^2 = $   ?

$(49x+6y)(49x-6y)$ $(7x+6y)(7x-6y)$ $(49x+36y)(49x-36y)$ $(7x+6y)^2$

$49 \,- 144\,x^2 = $   ?

$(49+12x)(49-12x)$ $(7+144x)(7-144x)$ $(7+12x)(7-12x)$ $(49+144x)(49-144x)$

$64\,y^2 \,- \,x^2 = $   ?

$(8y-x)^2$ $(8\,y^2+\,x^2)(8\,y^2-\,x^2)$ $(8y+x)(8y-x)$ $(8y+x)^2$