I have a frequency Band Pass responce at 44MHz and 6MHz bandwidth. how can I calculate its 3dB lower and 3dB higher frequency. means what will be the frequency ant 3dB lower point and 3dB higher point at 44MHz and 6MHz bandwith.

 

If 44MHz is the center frequency, then the 3dB points are +/- 3MHz on either side - 41MHz and 47MHz. It's a bandpass filter, so both 3 dB points are negative.

 

Actually, the center frequency of a bandpass filter is the geometric mean of the upper and lower -3dB frequencies. In this case, they can be calculated to be 41.102Mhz and 47.102MHz.
For more info, Google "bandpass geometric mean".