Creating a formula for a set of data in excel

Dear all,
I have a set of x y z data and I would like excel to give me a closed form formula
z=f(x,y) for the set of data such that the formula can reproduce all z data by inserting
the corresponding x,y . I was wondering if you could provide me a way I can perform that
either in excel or any other tool you are familiar with.
Thanks in advance.
Mohsen

AttachmentSize
x-y-z data.xlsx10.89 KB

had attached fle for the same in sepratethread

please check the excel file for the same in seprate thread

XY X Y

XY X Y Z
=(x&Y)
here i had used =CONCATENATE to merge xy and than did vlookup for z value

PUT X Y DATA HERE
X y formula =VLOOKUP(A13&B13,A2:D8,4,0)
1644875.014 -267323044.6 6
2281609.532 -279459379.9 7
3147342.811 -288930548.2 8
4009292.837 -296525882.8 9
4975113.619 -302751050.5 10
6223799.092 -307946051.3 11
7415536.599 -312346552.1 12