You may, first, generate points in a rectangle, for example, between -100 and 100 along "x", and between 0 and 10K along "y", and second, pick the points inside the parabola y=x*x --- note, about 2/3, two thirds, of all generated points are inside because the area inside is 2/3 of the total area, so, you may use the excel attachment, good luck.
p.s. if you fix a run (for the random points generated) then you may plot it inside the parabola