Concatenate & substitute problem

I have a file of 900 records and I need to take the number data in two cells and create one number in a cell and then I need to insert a "X" in the middle of the two former numbers. I have used concatenate to create the one cell but I cant figure how to insert the "X"

12 6 126 need the result= 12 X 6
5 13 513 need the result= 5 X 13

I am hoping someone can show me a way to get this result before I do this one at a time 900 times

Thank you

Thank You I knew I was overthinking it
Cheers!!!