how can i delete old videos on my iphone 4 as it wont let me delete it off the device?

My iPhone 4, iOS 7 and had not a lot of storage left, was previously used by my father who took a lot of videos. However, when I went to erase them I couldn't delete the videos, one which was about 11 minutes long. Do you have any way of deleting these permanently as my father already has back-up, it's just the case of me deleting those videos. When I go to delete them, it didn't have the delete sign on the bottom right hand corner, which confused me. Any advice?