Fermat's Little Theorem :
a^{p }≡ a (mod p)
According to Modular Arithmetic a ≡ b (mod n) if their difference (a-b) is an integer multiple of n ( n divides (a-b) )
So ( a^{p} - a ) is an integer multiple of p , now as a is not divisible by p so definitely ( a^{p-1 }-1) is an integer multiple of p .this simply means if we divides a^{p-1 } by p , the remainder would be 1 .... a^{p-1 }modulo p = 1
put the values in the formula. p=17 so p-1 =16 .