maths and stat

Confusion of permutation and Combination.....
I really dont know how to distinguish the case between two of them when doing the textbook exercise....
For example,,,
1) A student has 15 different books.In how many ways can he arrange 12 of them on a shelf if there is no restriction?

I use the method of 15C12 to answer it.However,I should use the method of permutation instead to attain the answer.

Another example,,A sample of 5 cars is selected for a destructive test from a group containing 7 small,4 medium and 6 large cars.How many samples are possible with no restriction?

And this time,I use the methods of permutation....however,this time is wrong....
I really confused....