Explanation:
it seems we are dealing with multiplication, as the distances between the numbers get bigger and bigger.
and then I see
7 × 2 = 14
14 × 2 = 28
28 × 2 = 56
ok, so the sequence is created by always multiplying by 2.
a1 = 7
a2 = a1 × 2 = 7 × 2 = 14
a3 = a2 × 2 = a1 × 2 × 2 = 7 × 2 × 2 = 28
an = an-1 × 2 = a1 × 2^(n-1) = 7 × 2^(n-1)
a14 = 7 × 2¹³ = 7 × 8192 = 57,344