Something has been done to Y to make it X. To go back from X to Y, do the opposite. Here, Y is multiplied by 0.85 and then 3 is subtracted. The opposite of "multiply by 0.85" is "divide by 0.85". The opposite of "subtract 3" is "add 3". And, of course, you must reverse the order of those operations. So first add 3 to both sides, the divide both sides by 0.85.