Factorise the following algebraic expression.

${\left(2x+3y\right)}^2-16z^2$

Factorise the following algebraic expressions.

${\left(x+y\right)}^2-{\left(x-y\right)}^2$

Factorise the following algebraic expression.

$4{\left(x-y\right)}^2-{\left(x+y\right)}^2$

If $9{\left(x+y\right)}^2-16{\left(x-y\right)}^2=(kx-y)(ly-x)$, then find the value of $k+l$.

Factorise the following algebraic expression.

$x^2+y^2+2xy-z^2$

Factorise the following algebraic expression.

$4x^2+12xy+9y^2-z^2$

Factorise the following algebraic expressions.

$25-a^2-2ab-b^2$

Upon factorising the following algebraic expressions, we get $\left(x^2+y^2+xy\right)\left(x^2+y^2+kxy\right)$, then find the value of $k$.

$x^4+y^4+x^2{y}^2$

If the factors of the algebraic expression $x^4+x^2+1$ is $\left(x^a+x+b\right)\left(x^a-x+b\right)$, then find the value of $a+b$.

[Hint: Add and subtract $x^2$]