y=3x2-7 is a parabola with vertex at (0,-7) and y-axis (x=0) as the axis of symmetry.
Note where the parabola intersects the x-axis: 3x2-7=0, 3x2=7, x=±√(7/3), that is, (-1.53,0) and (1.53,0) approx. These points help you to draw the graph: (0,-7), (-1.53,0) and (1.53,0).