Perceptron quiz question 4

Let f(x) be the logistic function where f(x) = 1/(1+e^(-Wx-B)). The loss function L is taken as L = (y-(f(x))^2)/2. Here x, y are re constants and W, B are parameters that will be updated. In other words L is a function of W and B. Let the partial derivative of L with respect to W be ‘M’ and with respect to B be ‘N’. Calculate the value of ‘m’ and ‘n’.
If i find the derivative of L w.r.t W then why is a y term in the answer…since y is a constant and it will get eliminated.