First let us identify if the asset is a gain or loss. An asset is a gain if it contributes to the banks overall finance while it is a loss if it is a cost directly or indirectly.
Deposits of $300 million = Gain (+)
Reserves of $20 million = Gain (+)
Purchased government bonds worth $300 million = Loss (-) ---> This entails cost
Selling bank’s loans at current market value of $600 million = Gain (+)
Therefore adding up everything to get the banks net worth:
Stealth banks net worth = $300 M + $20 M - $300 M + $600 M
Stealth banks net worth = $620 million