1 litre = a cube of 10cm x 10cm x 10cm = 1000cm³
Your tank is 350mm x 350mm x 500mm = 61,250,000mm³
Divide by 1000 to convert mm³ to cm³ = 61,250cm³
Divide by 1000 to obtain the number of litres = 61.25l as previously indicated
BUT assuming your measurements are external and the tank material is 2mm thick, the tank will actually hold nearly a litre less.