hey im pretty stuck with this, I've got quite far though, I think..
then df/dx =
Then to find the stationary points, we must equate the differentials to 0, and I get to the 4 points:
(2y, -15), (2y, 8x+15), (-2y, -15), (-2y,8x+15)
Then by using the Hessian matrix
H = fxx fxy
Where fxx = 2x, fyy = 2y - 8x, fyx = -8, fxy = 8y
Then the determinant is
Then you put in the x and y co-ordinates and determine whether is a maxima or minima as < or > 0.
But I don't have any numbers...so if anybody can help or see where I've gone wrong