What's the value of the supply voltage in this circuit?
Well, using Thevenin and Norton we have the following circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

For \$\text{R}_\text{th}\$, we get:







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So:

$$\text{R}_\text{th}=\frac{\text{R}_1\text{R}_2}{\text{R}_1+\text{R}_2}\tag1$$

And for \$\text{I}_\text{th}\$, we get:







simulate this circuit

So:

$$\text{I}_\text{th}=\text{I}_{\text{o}_1}+\text{I}_{\text{o}_2}=\frac{\text{V}_\text{in}}{\text{R}_1}+\text{I}_2\tag2$$

So, we also know:

$$\text{V}_\text{th}=\text{I}_\text{th}\cdot\text{R}_\text{th}=\left(\frac{\text{V}_\text{in}}{\text{R}_1}+\text{I}_2\right)\cdot\frac{\text{R}_1\text{R}_2}{\text{R}_1+\text{R}_2}\tag3$$

Using the given values, gives:

$$6=\left(\frac{\text{V}_\text{in}}{3000}+3\cdot10^{-3}\right)\cdot\frac{3000\cdot3000}{3000+3000}\space\Longleftrightarrow\space\text{V}_\text{in}=3\space\text{V}\tag4$$