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. Is equal to zero, and let me put some parentheses here just to clarify things a little bit. All right, so let's see. Now it's just a little bit of algebra. Cosine of pi over four, we already know, is square root of two over two. Sine of pi over four is also square root of two over two. And let's see, what if we divide both sides of this equation by square root of two over two?