in ASCII table all alphabets are listed in consecutive locations, so you can assume a is 1 , then b is 2 , c is 3 , d is 4, e is 5, f is 6, g is 7 .......so on till z=26,
now given in question C +10, where C gives the value of starting address of the array , i,e value of G which is 7
so C+7 turns out to be 17 , i hope this helps