What I did not understand in the answer it says that effect of machine sale the $2,500 should be deducted and the effect if machine purchase $5,200 should be added
The machine they are selling is already included in the forecasts and therefore the forecast profit includes $2,500 profit from the machine they are selling.
When they sell the machine they won’t make the profit of $2,500 and so it needs subtracting.
The new machine is not I included in the forecast. It will make a profit of $5,200 and so the profit needs increasing by that amount.