Those two bytes represents a short (System.Int16) in bigendian notation (most significant byte first).
You can follow two approaches to get the decimal value of those two bytes. One is with the BitConverter class, the other is by doing the calculation your self.
BitConverter
// the bytes
var bytes = new byte[] {3, 204};
// are the bytes little endian?
var littleEndian = false; // no
// What architecure is the BitConverter running on?
if (BitConverter.IsLittleEndian != littleEndian)
{
// reverse the bytes if endianess mismatch
bytes = bytes.Reverse().ToArray();
}
// convert
var value = BitConverter.ToInt16( bytes , 0);
value.Dump(); // or Console.WriteLine(value); --> 972
Calculate your self
base 256 of two bytes:
// the bytes
var bytes2 = new byte[] {3, 204};
// [0] * 256 + [1]
var value2 = bytes2[0] * 256 + bytes2[1]; // 3 * 256 + 204
value2.Dump(); // 972