For the polynomial $p(x),$ if $p(7)=0,$ then $\ldots$ is a factor of $p(x).$

$x-1$ is one of the factors of $p(x) = x^3 - 3x^2 + 7x - 5$. Which of the following is a factor of $p(x) = x^3 - 3x^2 + 7x - 5$?

Factorise $25 x^{2}+9 y^{2}+64+30 x y-48 y-80 x$.

Factorise the following:
$16 x^{2}+4 y^{2}+9 z^{2}-16 x y-12 y z+24 x z$

Factorise $16 x^{2}+40 x y+25 y^{2}$.

If $p(x) = x^{4} - 2x^{3} + 3x^{2} - ax + 3a - 7$ is divided by $(x + 1)$,the remainder is $19$. Find the value of $a$. Also,find the remainder when $p(x)$ is divided by $(x + 2)$.