Suppose you have calculated the positions of the extrema and stored them in pos_extrema, and stored the y value at that minima in miny, then:
>> fplot(y,[-2 2])
>> hold on
>> fplot(miny, double(pos_extrema(1))+[0 0], 'r*')
>> fplot(miny, double(pos_extrema(2))+[0 0], 'r*')
>> hold off
Hint: there are four positions where y have identical minima. Two of those locations are at complex-valued x -- so there are two real-valued x with the same minima.
You should be able to deduce more about the nature of the positions from the fact that you have x^2 in the formula but not x.
