Let's say that y is equal to log base four of x squared plus x. What is the derivative of y with respect to x going to be equal to? Now you might recognize immediately that this is a composite function. We're taking the log base four, not just of x, but we're taking that of another expression that involves x. So we could say, we could say this thing in blue, that's u of x. Let me do that in blue. So this thing in blue, that is u of x. U of x is equal to x squared plus x.