Question

What's the voltage drop running through the parallel portion of the circuit?

Answer 1

The answer is 18V ( I attached my working for you):
Basically I found the resistance of the parallel and the series to give the equivalent resistance, from there I calculated the current.
I used that to calculate the voltage drop over the parallel circuit.

Answer 2

R 3/4 = (R3 * R4) / (R3 + R 4) = ( 9 * 18 ) /(9 + 18 ) = 162 / 27 = 6 Ohms
R e = R 1 + R 2 + R 3/4 + R 5 = 3 + 6 + 6 + 15 = 30 Ohms
I = U / Re = 90 V / 30 Ohms = 3 A
Finally for the voltage U 3/4 ( the parallel portion of the circuit ):
U 3/4 = 6 Ohms * 3 A = 18 V
Answer: 18 V