Firstly, you need to convert 75 inches into feet and inches. 75 inches in feet and inches is 6ft 3 inches.
Elliot was 5ft 11 then grew 5 inches.
There is 12 inches in a foot, meaning he will become 6ft 4.
You've then got to subtract 6ft 3 from 6ft 4, and this leaves you with 1 inch.
Therefore, Elliott's bed is 1 inch shorter than him.
Hope this helps :)