Solve Total Differential: Find \Deltaz for z(x,y)

[hex.halo]

Homework Statement

For:
z(x,y)=(x^2)y-3y
Find $$\Delta$$z and dz when x=4, y=3, $$\Delta$$x=-0.01, $$\Delta$$y=0.02

Homework Equations

The Attempt at a Solution

I got the partial derivatives:

df/dx=2xy, and df/dy=x^2-3

and solved dz=0.02

However, I don't know how to find $$\Delta$$z

 

[Dick]

Delta z is the true difference. z(3.99,3.02)-z(4,3).