If the difference between sum of digits at odd place and the sum of digits at even place is 0 or divisible by 11.

For 2_28  

Let at the plce of _ be a

sum of digits at odd place=8+a 

sum of digits at even place=2+2=4

Difference of sum=(8+a) - 4

                         =4+a(it should be either 0 or divisible by 11)   

so a will be 7 as 4+7=11.