$[13$ pt$]$ Let $f(x,y)=ln(x-y+1)-2x+2y$

a$.$ Suppose that the directional derivative at

a$.a=,b=(1,1)$ in the direction of unit vector $u=ai+bj$ is equal to $D_u(1,1)=-1.$ Determine the

b$.$ Tangent plane: value of a and $b.$

b$.$ Determine the tangent plane of the surface function $f(x,y)$ at $(1,1).$

$[13$ pt$]$ Let $f(x,y)=ln(x-y+1)-2x+2y$

a$.$ Suppose that the directional derivative at

a$.a=,b=(1,1)$ in the direction of unit vector $u=ai+bj$ is equal to $D_u(1,1)=-1.$ Determine the

b$.$ Tangent plane: value of a and $b.$

b$.$ Determine the tangent plane of the surface function $f(x,y)$ at $(1,1).$