Solve for :
Move the lone constant across. Only on its own is free of :
Note that appears twice in the problem with two different roles — once as a flat amount and once as a rate. Keeping them apart is the main hazard here.
Factor x out of the right-hand side.
Compute the gross-up multiplier.
Divide.
Substitute back to confirm. The fee term is , so the right side is
reproducing the left side. So .
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