I presume that C is the combination function nCr.
The question means the number of combinations of 5 objects out of 13 less the number of combinations of 5 object out of 8.
C(13,5)=1,287 and C(8,5)=56, so 1287-56=1231.
Expanding the functions and the expression: (13*12*11*10*9/5*4*3*2*1)-(8*7*6*5*4/5*4*3*2*1)=147720/120=1231.