Q:  Find the MUx and the MRS at (3,2) for
    1) u(x,y)=x3y and 2) u(x,y)=min(2x,y)

A: 1)  MUx=dU/dx=3x2y=3*32*3=81.
    MRS=MUx/MUy= 3x2y/x3=3y/x=3*3/3=3

2) MUx=0.  We know this because u(3,2)=min(2*3,2)=min(6,2)=2.  The person already has excess x.  Consuming more x would not increase utility as it is the y=2 that is keeping utility at a level 2.    Since we are on the horizontal part of the IC, we know the MRS=MUx/MUy = 0/+ = 0.