Reverse Engineer a Formula?

Use Goal Seek ...

To get the hang of using it, see below - its a previous posting of mine which provides an example / details
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Perhaps try something along these lines
to grasp a feel on using the Goal Seek* functionality ..
*It's on the Tools menu, not Data menu

Let's say we have the formula in B1:
=sum(A1+A2+A3+A4+33*(sqrt(200)))
and we have in: A2=22, A3=3 and A4=7

Assume we want to find out what the value in A1 should be for B1 to resolve
to a target value of say, 100, given the values in the other dependent cells
A2, A3 and A4

Select B1
Click Tools > Goal Seek

In the Goal Seek dialog,
ensure/make the settings as:

Set cell: B1
To value: 100 (< target value for B1)
By changing cell: A1

Click OK

The Goal Seek Status dialog will show that a solution has been found,
and we'd get the computed value in A1: -398.69

If we click OK, the dialog will dismiss and the value in A1 will be
retained.

If we click Cancel, the dialog will dismiss w/o any changes to the pre-run
state.
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