I have a form to enter parts used with the cost/price and kilometer

in another form, I would like to have a historical average price and kilometer off all a particular part

the question is:

1. how can I pick only 1 part from many parts, get the prices, then make 1 average price.

2. I also want to have a historical average kilometer used, based on the kilometer when they change the part. for example. maybe they change a radiator at 1000 km. then another at 4000 km. then the difference is every 3000 km. if another change is 9000, then the difference is (9000-4000)=5000, the average is (3000+5000)/2.

Please help. thank you for your attention.

Michael