-10 is equal to 5 subtracted from a number
Assume that the number is x
So the equation will be

we put x at first because in subtraction the number after the word "from" comes first
Now let us find x
We need to move 5 to the left side to find x, so add 5 to both sides

The number is
An inverse function is y = k/x
replace x and y with the given values:
6 = k/18
Solve for k by multiplying both sides by 18:
k = 108
I think it’s 20ft but I’m not really sure so I’m sorry if it wrong