> I assume the & & designates a variable in formula string?
& is the concatenation operator and is used to join strings.
"=AVERAGE(RC[-1]:R[" & x - 1 & "]C[-1])"
Would give you a string made up of:
the string "=AVERAGE(RC[-1]:R["
the value of x - 1
and
the string "]C[-1])"