77.94 - 77.94 equals zero because you are subtracting a number from itself.
Then you add 0 to 42.09 to get 42.09.
77.94 - 77.94 + 42.09 =
= 0 + 42.09
= 42.09
The sign of the answer is positive since you are adding a positive nunmber (42.09) to zero.
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