So what do we know, what is true if this tangent line is horizontal? Well, that tells us that at this point, dy dx is equal to zero. In fact, that would be true at both of these points. And we know what dy dx is. We know that the derivative of y with respect to x is equal to negative two times x plus three over four y to the third power for any x and y. And so when will this equal zero? Well, it's going to equal zero when our numerator is equal to zero and our denominator isn't.