Your question sounds like homework. You can find guidelines for posting homework on this forum here. If you have trouble with Matlab basics you may consider doing the Onramp tutorial (which is provided for free by Mathworks). If your main issue is with understanding the underlying concept, you may consider re-reading the material you teacher provided and ask them for further clarification.
As to your question: think about what characteristics a red pixel would have that other pixels wouldn't. What exactly would you be looking for?
In your follow-up, it would also help if you include an example image.
