So we then want to take the derivative of that to get us our second derivative. And so that's where that notation comes from. It looks like you have a d squared, d times d, although you're not really multiplying them, you're applying the derivative operator twice. It looks like you have a dx squared. Once again, you're not multiplying them, you're just applying the operator twice. But that's where that notation actually comes from. Well, let's first take the first derivative of y with respect to x.