Solved by albanD in post #2 
              
              
                Hi, 
Your network corresponds to the following function: f(x) = y = x^2 and df(x) = 2*x. 
Your backward pass will compute dL/dx = dL/dy * dy/dx = 2.0 * df(x) = 2.0 * 2 * x = 4*7 = 28.