Find the emf that makes a branch quiet

A 15 V source in series with 3 Ω and a second source of adjustable emf E₂ in series with 4 Ω connect across a 6 Ω load. Both positive source terminals face the upper output rail.

At what E₂ is the second branch current zero?

Organize the givens & assumptions

First source: 15 V with 3 Ω
Second source: E₂ with 4 Ω
Load: 6 Ω

Settled lumped DC circuit, ideal connecting wires, positive constant Ohmic resistances and no changing external magnetic flux. Sources are ideal except for explicitly specified series resistances.

A $15\ \mathrm V$
B $6\ \mathrm V$
C $5\ \mathrm V$
D $10\ \mathrm V$

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