You can delete the partition that is to the right in disk management and then expand the remaining partition into the unallocated space, BUT to do that it must be a GPT formatted disk to use the full size (over 2.2Tb) as one volume. Make sure that is the case, and that it is not MBR formatted. Check by right clicking in disk management and looking at the disk properties under the volumes tab and it will list the partition style. It should say GUID Partition Table (GPT) and NOT Master Boot Record (MBR).
As mentioned, you can backup your data, then delete the adjacent partition, then combine them. Or use some free partition software which can directly merge two adjacent partitions.
you can see this youtube video tutorial about how to merge two partitions: http://www.youtube.com/watch?v=yLX_9xwE_rs
IN W7 IT HAS 2 PARTITIONS WANT TO MAKE AS ONE
what's the two partitions? both of them are two partitions ? or just one partition and another is unallocated space?
If two of them are partitions you could merge them with some the disk mangement or other partition software such as AOMEI Partition Assistant,
If one is partition and another one is unallocated space, you should notice that, maybe your disk format is MBR, and as we know the full use of the MBR disk is just 2.2TB (as "RealBeast" said). To solve this you need to convert it to GPT disk, which support larger disk space to use.
To convert it to GPT disk, you could backup your files then delete all the partitions, and initialize the disk to GPT (just right-click your disk and select "Initalize" in disk mangement) or if you do not want to waste time on backing up the files, you could use other tools to convert it to GPT without lossing data directly. Here I find a video about this hope it could help you http://www.youtube.com/watch?v=hlYjNbiVB-0