The expression 0.8w + 3.6 factored is

4 could be considered a common factor
0.8w + 3.6 =
4(0.2w + 0.9)
or you could call 0.4 the common factor
=0.4(2w+9)
= (2/5)(2w+9), if you want to eliminate decimals. 0.4=4/10=2/5