To do this in your head:
voltage at a = voltage at b
you have 3 x 680R in series with a 340R
If you make 340R equal to 1 unit of resistance, you have 3 x 2 units plus 1 unit = 7 units
This means you have 10v divided by 7 units = 10/7 volt per unit = 1.42volts at point a and b
If a and b are different, you work out the parallel combination of the two and divide the whole series resistance of the chain into the voltage and then multiply the result by the lower resistance.