Based on chargaff's rule, the percentage of cytosine in the dna of the bacterium s. lutea in figure 12-3 should be around what? the answer is 36.6%, but i don't know why. what's the reason why the answer is 36.6%?

You have to remember Chargaff's rule which is pretty simple:
A always pairs with T; G always pairs with C
So look at your table. 
Human: you know 2 numbers: G is 19.9 and T is 29.4. By Chargaff's rule you can fill in the other 2 so you'd have:
A (29.4), G (19.9), T (29.4), C (19.9)
I'll skip to the last one on the bacterium:
Here you only know one. But, you really know 2: 
A is 13.4 so T must also be 13.4. This adds up to 26.8.
100-26.8= 73.2 (which is the amount of G+C); so divide this by 2 and you get 36.6 - so 36.6 G and 36.6 C!
Just remember who pairs with who...will never let you down!

The answer is c : ) hope this answered your question!