Struggling with this textbook question:

"The equations

x+2y+3z+4u=-3

x^{2}+y^{2}+z^{2}+u^{2}=10

x^{3}+y^{3}+ z^{3}+u^{3}=0

define u as a function of y if x and z are eliminated. Find du/dy when x=1, y=-1, z=2, u=-2"

The problem I am having is why they give three equations? There must be a reason and I am missing it.

Is anyone able to explain just what the question is asking for?

Thanks

Laura