This is really easy, but there is one big thing to remember:
THE SIZES MUST BE THE SAME!
Check it out:
Let's add these guys:
We just add the entries in each spot...
That's it! So, what do ya think -- easiest thing on the whole site?
Here's a subtraction guy -- same idea:
Notice what's going on with the sizes... The matrices you add or subtract have to match up!
And the answer is the same size.