And so what x value, when I take the sine of it, and remember, where the angle, if we're thinking of the unit circle, it's going to be in that first quadrant. X is an angle in this case right over here. Well, that's going to be, once again, pi over four. So this tells us that x is equal to pi over four when y is equal to pi over four. And so we know that this is pi over four as well. So let me just rewrite this because it's getting a little bit messy. So we know that five times cosine of pi over four minus dy dt, the derivative of y with respect to t, which is what we want to figure out, times sine of pi over four is equal to zero.