To find the inverse of any function, switch the x's with y's and vice versa.
The current equation given is 
Switch the
with
and switch the
with 
After doing so, we'll have the following equation:

Now, let's move the variables around so that y is isolated on the left side again.
Subtract both sides by 4y

Subtract both sides by x

Divide both sides by -4

That is the answer.
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I think the interquartile range is 13. Hope it help!