Step 01 -> Right click with your mouse on the 'Eyedropper Tool' and move you mouse to 'Measure Tool.'
Step 02 -> Place your mouse on the left side, click on your mouse and hold it like that.
Drag the line to the other end of the page, then release your finger from the mouse.
Step 03 -> Go to Image -> Rotate Canvas -> Arbitrary.
The program will automatically tell you the value the page needs to be rotated. All you need to do is to hit on 'OK'.
*Note: Your number doesn't necessarily have to be the same as mine! As long as it comes close, it's fine.
And this is how it looks like right now.
All that there's left to do is to crop the page. Please check the next tutorial for that.