plz help in this problem


thanks.......

 

The first circuit is a series circuit. This means that all the elements have the same value for current. Work this out by using the resistance of R3 and the p.d. across it (all given in question) then use that current value and ohms law to work out the rest.

 

The only thing you have to know here is Ohm's Law.
\(\Delta V = R I\). The current I is the same in R1, R2 and R3. You have \(I = \frac{4V-(-4V)}{8\Omega} = 1A\). Since I is the same on all the branch, \(R_1 = \frac{12V-4V}{1A} = 8\Omega\) and \(R_3 = \frac{-4V-(-8V)}{1A} = 4\Omega\). Not more complicated than that.