Hello, thank you for taking the time to post your question!
In order to convert this one over you just want to scale by the ratio of 12 inches per foot
7 feet x 12 inches/foot = 84 inches
So the underlying here is f(x) = 12x, where x is the number of feet that you are trying to convert over into inches
I hope that helps get you moving in the right direction! Feel free to reach out if you have any additional questions beyond that :)