Changing this image to be transparent

[tm2383]

I work as a teacher for the visually impaired (VI) and I need to make this image of a special protractor transparent, leaving the black lines and numbers, so that I can demonstrate it being used to measure angles on an interactive screen. The protractors which come with the screen software aren't VI ones. 
I have looked at tutorials, but haven't successfully managed to do this. How would I go about doing this?