Voltage divider reverse formula
It will look something like this: 

$$V_{OUT} = ADC\cdot \frac{V_{REF}}{1024} \cdot (1 + \frac{R_1}{R_2}) $$

Where \$ADC\$ is a value (between 0...1023) read by Arduino ADC. and \$V_{ref}\$ is an Arduino ADC reference voltage.  

Or to answer your question directly 

$$V_{CC} = V_{ADC} \cdot (1+\frac{R_1}{R_2}) = 2.824V \cdot8.5 = 24V $$