Explanation:

Given series is 7,5,14,7,(),9,56,11
 1st,3rd,5th,7th
 7,14,(),56,
 2nd ,4th,6th,8th
 5,7,9,11
 there is two series
 7 ×2=14
 14 ×2=28
 28 ×2=56
 multiply by 2
 2nd ,4th,6th,8th
 5+2=7
 7+2=9
 9+2=11
 added 2
 28 is filled in the series