Christopher weighs 211 pounds and has a bmi of 28.5. how many whole ponds must be gain or lose to get into the healthy bmi range?

BMI = (pounds * 703) / (height^2)

so in the information you are missing the height
let's find it :)
first plug in what you know:
28.5 = (211 * 703) / (height)^2   then solve for (height)
28.5 (height)^2 = (211 * 703)     multiplied both sides by (height)^2
(height)^2 = (211 * 703) / 28.5   divided both sides by 28.5
(height) = square root [(211 * 703) / 28.5]  square rooted both sides
(height) = 72.143 inches

The healthy BMI is about 25
so 25 = (pounds)(703) / (72.143^2)
25(72.143^2) / (703) = (pounds)  divide by 703, multiply by (72.143^2)
pounds = 185

211 - 185 = 26
Therefore Christopher would have to lose 26 pounds.

*Note: If the BMI you wanted to use was not 25, then just replace it like this
(pounds) = (BMI)(72.143^2) / (703) = goal weight