Finding Vc in this simple circuit
The way you've drawn it, R1 and R2 form a voltage divider across the 12V supply. Since both are the same resistance, the voltage at the node that forms the '-' terminal of Vc is 6V, half the supply voltage.

With Vc open circuit, no power flows through R3, and so no voltage is dropped across it. Therefore, it's at the same voltage as the negative terminal of V1 - 0V.

So, the voltage across Vc is 6V - 0V = 6V.