Someone else commented the pattern. The answer is not 14 or 15.
Hint
There is no function. You won't find the answer by trying different combinations math operations on the values. You need to think more outside the box.
Hint #2
Ignore the values.
Hint #3
Focus on just three numbers. How can 13 and 21 produce 7?
What do you mean you're unable to install it? That screenshot is not what the package installer looks like.
Android needs the same sort of hardware-specific configuration that other distros need. It's just that the phone manufacturer takes care of that for you
Because her face is lit from the top by a point light source
spoiler
Yup!