If I understand this challenge correctly, it is to use an editing tool to ensure a straight line is visible within the photo. I struggled with this, so instead, I took a photo of some artwork in my office. The framed photo in the middle has driven me crazy in trying to hang it as straight as I can. It was when I recognised that the frame is straight but the photo subject (part of our local Busselton Jetty) leans to the right I understood the problem!