If the derivative value of the function y = x ^ 4 + ax ^ 2-4 / 3A at x = a is 0, what is the constant a

If the derivative value of the function y = x ^ 4 + ax ^ 2-4 / 3A at x = a is 0, what is the constant a

y'=4x^3+2ax,
y'(a)=4a^3+2a^2=0;
A = 0 or = - 1 / 2