$\text { Let } f ( x , y ) = x ^ { 2 } y ^ { 2 } - 1$

Short Answer

$\text { Let } f ( x , y ) = x ^ { 2 } y ^ { 2 } - 1$ Compute $\frac { f ( - 1,1 + k ) - f ( - 1,1 ) } { k }$ for k ≠ 0.
Enter your answer as a polynomial in k in standard form.