And if you're wondering where this notation comes from for a second derivative, imagine if you started with your y and you first take a derivative, and we've seen this notation before. So that would be the first derivative. Then we want to take the derivative of that. 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.