Now what is du going to be equal to? I will do it in magenta. du is just going to be the derivative of x to the fourth plus 7 with respect to x, so 4x to the third plus 0 times dx. I wrote it in differential form right over here, but it's a completely equivalent statement to saying that du, the derivative of u with respect to x, is equal to 4x to the third power. When someone writes du over dx like this, this is really a notation to say the derivative of u with respect to x. It really isn't a fraction in a very formal way, but oftentimes you can kind of pseudo-manipulate them like fractions. So if you wanted to go from here to there, you could kind of pretend that you're multiplying both sides by dx.