$(2pts)$ Find the equations $of$ the cross$-$sections $ofz=y^2-xy$ with fixed $x=0,1,$ and fixed

$y=0,1.$ Sketch them $in$ the appropriate coordinate planes.

$(3pts)(a)$ Sketch the level$-$one contour $of$ the surface $z=y^2-x^2.$

$(b)$ Sketch the level$-$one contour $of$ the surface $z=1-x+2y.$

$(c)$ Sketch the contour $of$ the surface $z=x^2+y^2$ that passes through the point $(1,1),$ and

clearly label its level.