We don't know what x is, and we don't know what the derivative of y with respect to t is. And this is what we need to figure out. So what would x be? What would x be when y is pi over four? Well, to figure that out, we can go back to this original equation right over here. So when y is pi over four, you get, let me write it down, sine of x plus cosine of pi over four is equal to square root of two. Cosine of pi over four, we revert to our unit, or we think about our unit circle.