RESISTANCE

Generally R=\frac{\rho (length)}{Area of cross section}

Apply this here

which lenth need 2 take y c?

Length is to be taken along the direction which current will flow when the resistor is connected to a battery

from A to B

then length= b

see current is flowing from A to B.so the length is along the current flow direction bcos they are the terminals.current will flow from positive to negative potential.