Question 717353
A number is divisible by 9 if the sum of the digits is divisible by 9. If we call the missing digit "n" then the sum of the digits, 8+8+n+1+2, must add up to a number which is divisible by 9.
Simplifying 8+8+n+1+2 we get 19+n. "n" must be a valid digit, 0 through 9. Which one must it be if 19+n has to be divisible by 9? There's only one possible answer. I'll leave it up to you to finish. Just try different digits for "n" until 19+n is divisible by 9.