you said what wrote:
"relay meet" in XC?
We actually did a 4 x 1.5 relay in XC way back when. It was big in the early 90s, but who knows if they still run it.
As for the original question, assuming it's a 5k, you're probably going to be 20 seconds slow. Maybe as much as 45 since you aren't acclimated. Be very careful with hills. Altitude makes oxygen debt recovery very, very difficult, so charging up an early incline is a great way to ruin your race.
(If it's less than a 5k, the altitude effect will be less. Probably more like 3-4 seconds per mile instead of 6-7.)