So w prime of t. Well, the derivative of a constant with respect to t is zero. And the derivative of this with respect to t, and this comes straight out of the fundamental theorem of calculus, this is going to be, this is going to be r of t minus d of t. Where once again, this is a function. We have t as the upper bound. And so we have whatever here that we were integrating, but it's now going to be a function of t. And so in order for this to be equal to zero, we need to figure out when does r of t minus d of t equal zero. And lucky for us, we've already typed in r of t minus d of t. We've defined that as y sub one on our calculator. So let's go back here. And now I can use the solver.