Does your answer still have dy/dx in it? Substituting the known expression for that, followed by a little algebra, might fix it up for you.
Does your expression still have x^2 + 4y^2 in it? Substituting the known value for that might fix it up for you.
Did you find the second derivative by differentiating the expression for the first derivative, using the quotient rule? Sometimes, it is more convenient to differentiate the original expression twice, using the product rule.
Just some hints. Without you showing your work, it's hard to know where you may have erred, if, indeed, you did.