A force of −Fk^ acts on origin, then the torque about the point (1,−1) is :

τ→=r→×F→ Force acts on origin so r→ is position vector of origin wrt. (1,−1) sor→=−i^+j^,F→=−Fk^τ→=(−i^+j^)×(−Fk^)=−Fi^+Fj^