For this question:
The base of a particular solid is the region bounded by the parabola y^2=4x between its vertex (0,0) and its latus rectum. Find the volume of the solid if every cross-section perpendicular to the x-axis is an equilateral triangle with one side in the base of the solid.
I do not get the solution attached at the circled bit. Could someone please show me how that line was obtained from the above lines?