Explanation:

The number of combinations of 3 vowels and 2 consonants should be 4×6=24 instead of 4+6=10. 
Since you are considering combinations, each set of 3 vowels and each set of 2 consonants form a combination, 
so you need to multiply and not add.
The combinations are ({I,O,E},{N,V}),({I,O,E},{N,L}),….
You can try writing out all 24 combinations as an exercise.
Then you will get 24×120=2880.