This is just the same as asking you what number when subtracted by 20.5 will give you an answer of 8. The answer would obviously be 8+20.5=28.5 so w=28.5.
We can also do it in a more algebraic way. By adding 20.5 to both side of the equation which won't change the value.
w-20.5+20.5=8+20.5
w=28.5
Any questions just ask me in the comments of this question :)