What is the reading on voltmeter V1?

The top branch contains a $3 \, \Omega$ resistor and a $6 \, \Omega$ resistor connected in series.
The total resistance of the top branch is $R_{top} = 3 \, \Omega + 6 \, \Omega = 9 \, \Omega$.

Ammeter A1 is in series with these resistors and reads $6 \, \text{A}$. So, the current through the top branch is $I_{top} = 6 \, \text{A}$.