set mainString to "87654321"
set n to 3
set m to 4
set c to "5"
set sub to "654"

put characters n + 1 to  n + m of mainString

put characters  n + 1 to end of mainString

put characters first to penultimate of mainString

set characterOffset to offset of c in mainString
put characters characterOffset to characterOffset + m - 1 of mainString

set subOffset to offset of sub in mainString
put characters subOffset to subOffset + m - 1 of mainString