From the visible arrows I assume this is a screenshot from the 3D view. Is that where you want the sides to be equal? If so: Select all, that should move the total to the center. To get equal spaces on all sides you can move the pane-separators to change the size of the 3D view pane. And switch of the visibility of the arrows (Preferences, 3D navigation arrows.)

But... why not take a screenshot with an area selection? I'm not sure what system you are on but on my Linux install I can choose how to take a screenshot: full screen, active window, or area selection.

And alternatively you can make it even easier: simply resize the image with an image editor like GIMP (Change the width of the canvas and move the image to the left. That will cut of a piece on the right.)