(a) Find and classify the nature of the critical points for $(x,y)= 3x? +9y? - 3xy-x+ 2021. (b) Use the method of Lagrange Multipliers, find the maximum and minimum values of f(x,y)=x²-2y? +a subject to the constraint x + y =4. (c) Given g(x, y, z)= g(aw-au v+2u,v+3w) is a differentiable function, determine og og og . ди ди ду +