Let a be the x-value of where the normal intersects x^2. Express m, the gradient of the normal line, in terms of a. Then (at least if you do a sketch) you can see that c, the height of where the normal intersects the y-axis, will be equal to a^2 minus ma, which you should find is equal to a^2 + 1/2. (Which is at least 1/2)