Let r = x2 + y - z and z3 -xy + yz + y3 = 1. Assume that x and y are independent variables. At (x, y, z) = (1, -1, 1), the value (correct to two decimal places) of \(\frac{{\partial {\rm{r}}}}{{\partial {\rm{x}}}}{\rm{\;}}\) is _________.
1
3
2
3.5
3
4
4
2.5