Apair of shoes bought by your friend amounted to (3x-5y+2z)² dollars. how much does it really cost?

We need to multiply 3x-5y+2z by itself.
I break it down by each term. We'll first do 3x. It is positive.

3x(3x-5y+2z)
3x×3x=9x²
3x×-5y=-15xy
3x×2z=6xz
9x²-15xy+6xz
Now -5y.

-5y(3x-5y+2z)
-5y×3x=-15xy
-5y×-5y=25y²
-5y×2z=-10yz
25y²-15xy-10yz
Now 2z.

2z(3x-5y+2z)
2z×3x=6xz
2z×-5y=-10yz
2z×2z=4z²
4z²+6xz-10yz

Now we need to add all of them together.
(9x²-15xy+6xz)+(25y²-15xy-10yz)+(4z²+6xz-10yz)
We need to add like terms. 
9x²+12xz+25y²-20yz+4z²-30xy
Now order them by degrees.

25y²+9x²+4z²+12xz-20yz-30xy is the cost.

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Where is the rest of the question