The way you figure this out is by taking the length (5/6) and subtracting the width (3/8). in order to subtract fractions, they need a common denominator. the number 24 works well for this as 8*3=24 and 6*4=24. in order to keep everything equivalent, we can only multiply by 1, but we can rewrite 1 by taking a number over itself, such as 3/3 and 4/4.
so if we take 5/6 and multiply by 4/4 (which is still equal to 1) we get 20/24.
next we multiply 3/8 by 3/3 (which is still equal to 1) we get 9/24.
now that we have a common denominator, we can simply subtract them. 20/24-9/24=11/24
this means our answer is C. 11/24 ft