Current and voltage dividers with multiple sources
For the first circuit diagram, did you notice that the 3 &Omega; resistor is short circuited, and hence the + terminal of the ideal 18 V source is connected to the top of the 2 &Omega; resistor? Knowing this, you can use the voltage divider on the 2 &Omega; and 10 &Omega; resistors : $$v_1=\frac{2}{10+2}\times (18-6)\:\text{V}=2\:\text{V}$$

For the second circuit diagram, the 36 mA source only feeds the parallel 1.2 k&Omega; and 3.6 k&Omega; resistors. The remainder of the components are just hanging off the lower  terminal of the 36 mA source, and have no return path to the top of this current source (easy to see if you re-draw the circuit and move the 3.6 k&Omega; resistor up, to join the node just below the 36 mA source). Hence the current divider can be used: $$i_1=\frac{3.6}{1.2+3.6}\times 36\:\text{mA} =27\:\text{mA}$$