I have two T4 slips from the same company + 1 Releve1 for that company. releve 1 is the sum of the two T4 slips. how do I add this in this form? I can't edit box C

I have two T4 slips from the same company + 1 Releve1 for that company. releve 1 is the sum of the two T4 slips. how do I add this in my form? I can't edit box C

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