Verified answer

Then the sentence is:

x = (2z²)² * (x + y).

The key is to notice that S is a sentence that maps ℤ³ to ℤ³ with the coordinate relationship:

third coordinate = square of second coordinate * first coordinate.

Just do a direct substitution in that sentence, replacing x by x+y, y by 2z^2 and z by x and you get S(x+y, 2z^2, x) = (2z^2)^2 * (x + y) = 4z^4(x + y) and there is no great deal of point of expanding the bracket since that won't give you something you can simplify. Hope that helps, and is clear enough.