the thing im confused about if c is a constant? as soon as you differentiate it, it cancels?? coz c isnt even in df
umm i tried that df=0 if 2xy+y+z=0, x^2+x+2y+z+3=0, x+y+3=0?? but i solved for each x y and z and got really dumb answers, since its a quadratic..