There are genealogy programs who will give you an answer but I like to do it the old fashioned way.
Get out a pencil and paper.
X (joint great grandparent) X (grand parent's sibling -great aunt)_______ X X (your mother)_______________________grandson
Now go down from the common ancestor: siblings, 1st cousins, 2nd cousins, etc. If the generation is 'off' go to the closest generation and use the 'removed' term. This grandson is YOUR 2nd cousin once-removed.