How to use Millman's theorem to solve this question:find the voltage value
Homework, only guidance will be given.

The right half has no connection to the left half except R3. Thus there's no current in R3 => Vc=Va+5V. The situation would be different if there were for ex. a GND connection in the right half of the circuit, but there isn't, so the voltages at points b,c,d and e ride on what sources V1 and V6 generate to point a.

Calculate Va with Millman's formula. Use source voltages V1 and V6 and resistances R1 and R2. Then add 5V. That's your Vc calculated by applying Millman's theorem as a part of the calculations.