Question about doing impairment test

P2 study text , exam question bank , question 4 – Camel telecom part A

– the asset paid 344m, the market value 370 m , useful life 10 yrs.
-the difference become the government grant : 26m

– the amortization : 344/10 = 34.4m less 26m/10 =2.6m , so the net : 31.8m

market value at yr end : 335m

my question is to do the impairment test at year end, why using the net book value of the asset at the yr end is 309.6m (344m-34.4m) (from the answer) ?

can i use this net book value is not 312.2m (344m -31.8m) ?

thank you very much.