-0.9 + 3.2

The sign of the first term is negative
The sign of the second term is positive.

So the signs are different.

When the signs are different, we subtract their absolute values
as we did in the 3rd or 4th grade:

3.2
0.9
2.3 

Then we take the sign of the larger of the absolute values,
which is 3.2 and it is positive so the answer will be positive:

Answer: +2.3 or just 2.3

Edwin