Plot3D[Sin[x*y],{x,-2,2},{y,-4,4}]

Plot3D[{36-x^2-y^2, x^2 + y^2},{x,-4,4},{y,-4,4}]

ContourPlot3D[{y-(x^2)==0,z-(x^2)==0},{x,-3,3},{y,0,4},{z,0,4}]