$23-40.$ Analyzing critical points Find the critical points of the following functions. Use the Second Derivative Test to determitre $($if possible$)$ whether ench critical point corresponds to a local maximum, a local minimum, or a saddle point. If the Second Derivative Test is inconclusive, determine the behavior of the function at the critical points.

$23.f(x,y)=-4x^{2}8y^2-3$

$24.f(x,y)=x^4{y}^4-4x-32y10$

$25.f(x,y)=42x^{2}3y^2$