Lucas cleans a total of 3 floors * 3 windows/floor = <<3*3=9>>9 windows.
The windows cleaned earn Lucas 2 dollars each, so Lucas earns 9 windows * $2/window = $<<9*2=18>>18
For taking 6 days to clean, Lucas' father subtracts 6 days / $3/day = $<<6/3=2>>2
In total, Lucas' father will pay him $18 - $2 = $<<18-2=16>>16
#### 16