Start by putting the arrangement of books below on the 
shelf so that there are exactly 2 math books between any 
two physics books there.

     __PMM__PMM__PMM__PMM__MP__

(M stands for a math book and P for a physics book, and the 6 blanks
are gaps or places where we can insert the remaining math books

All 5 physics books, P's, and 8 of the 13 math books, M's are on the 
shelf.  So we have to insert the remaining 5 math books, M's, into
some of the 6 blanks.   

That's the number of partitions of 5 things into 6 blanks,
(which includes putting no M's in some or even 
all but 1 space, and all 5 in that space.)

The number of partitions of n indistinguishable things into r places. 
[which includes putting 0 in some places, and extreme cases where all n
things are in 1 place.]

is 

(n+r-1)C(r-1), where n=5, r=6

(5+6-1)C(6-1) = 10C5 = 252 ways to arrange the books.

Answer: 252

Edwin