Answer:
You have to align the decimal points. Imagine 4 as 4.0 and add
4.0
+ 2.6
_____
6.6
if you misalign the decimal points then your answer is wrong.
If it was 4.001+2.6 you would still have to align like shown below
4.001
+2.6
_____
6.601
The decimal points have to be aligned.
If you commented the dropboxes I could help with them.
Hope this helps
Explanation: