SageMath is a free open-source mathematics software system. You can use a SageMath cell online by going here.
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var("x,y")
plot3d(3*x^2+y^2, (x,-1,1), (y,-2,2))
xvar("x,y")
plot3d(3*x^2+y^2, (x,-1,1), (y,-2,2), mesh=True, opacity=0.6)
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var("x,y,z")
implicit_plot3d(3*x^2+y^2-z, (x,-2,2), (y,-2,2), (z,0,4), mesh=True, opacity=0.5, color="green")
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var("x,y")
contour_plot(3*x^2+y^2, (x,-2,2), (y,-2,2), contours=5, labels=True, fill=False)
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var("x,y")
contour_plot(3*x^2+y^2, (x,-2,2), (y,-2,2), contours=[1,2,3,4], fill=True)