Startled by a notation of operator for combining two signals
V1 is 100mV attenuated by 3.3 dB and +3.3 dB = a ratio of 1.462.

Therefore V1 becomes 100mV/1.462 = 68.4 mV (as you have said).

V2 is 400mV reduced by 0.902 to 360.8 mV (as you have said)

Here's where the problem lies - these are uncorrelated noise sources and have to be added using pythagoras....

So, the output from the summer is \$\sqrt{0.0684^2 +0.3608^2}\$ = 367.2 mV

This is then increased by 4 dB (ratio of 1.584) to 1.584 x 367.2 mV = 582 mV.