We don't actually know what the problem originally was, so we don't know what the error was or what the correct answer should be.
If the numbers on the left are given, and the problem is to find the number on the right, then ...
Paolo added 0.2 instead of subtracting it.
-0.43 -0.2 = -0.63 . . . . . correct answer
_____
If the left and right numbers are given and the problem is to find the one in the middle, then ...
Paolo incorrectly calculated 0.43 -0.23, getting -0.2 instead of +0.2.
-0.43 +0.2 = -0.23 . . . . correct answer
_____
If the problem is to find the term on the left, then ...
Paolo incorrectly added 0.2 and -0.23, getting -0.43 instead of -0.03.
-0.03 -0.2 = -0.23 . . . . correct answer
