Explain why the functions differentiable at the given point.

$f(x,y)=2+xln(xy-7),(2,4)$

The partial derivatives are $f_x(x,y)=$ and $f_y(x,y)=,$ so $f_x(2,4)=$ continuous functions for $xy>$ and $f$ is differentiable at $(2,4).$

Find the linearization $L(x,y)$ of $f(x,y)$ at $(2,4).$

$L(x,y)=$