Hi,
This is simple:
12:5=(R1+R2):R2
This give you 7K & 5K Ohm, But with C_you must tell what you need to make/have?
Pls remeber; 12K on 12V delivers for you no practicable current-only voltage-potential!
How much energie/current do you need or with what kind of time constant?
K.
If you want to read the voltage from a 12V battery, you probably need a full scale ADC reading that represents more than 12V (a fully charged lead battery can go to more than 14V).