Partial Differentiation stationary points

hey im pretty stuck with this, I've got quite far though, I think..

f(x,y) =

then df/dx =

df/dy =

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

fyx fyy

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

Thanks