ok just a question about the working out for this problem,

I understand

4x^2 - 8xy - 9y^2 = -637

= 8x - 8y - 8x d/dx(y) - d/dy(9y^2)dy/dx

= 8x - 8y - 8x dy/dx - 18y dy/dx

but where does the + sign come from??? can anyone explicitly show where this comes from.. I think it must come from the product rule but I just cant see it...

(8x-8y)-(8x+18y)dy/dx