These matrices are the same size, so we can add them, no prob. We just add the entries in each corresponding place:

Both of these bad boys are 3 × 4 matrices, so they can be added. We just add the entries in each corresponding place:

Bzzzzt. Technical foul. These matrices aren't the same size, so we can't add them.

Two matrices of the exact same size? Check. Now we just line up each corresponding entry and subtract.

Subtract! Subtract like the wind!

Nope. Can't do it. We've got two different-sized matrices here, so subtraction is a bust.