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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:

[ row 1: 1 , 3 , 4  row 2: -7 , 0 , 5 ] + [ row 1: 6 , -3 , 11   row 2: 0 , 5 , -4 ]

 

We just add the entries in each spot...

[ row 1: 1 + 6 , 3 + ( -3 ) , 4 + 11  row 2: -7 + 0 , 0 + 5 , 5 + ( -4 ) ]


= [ row 1: 7 , 0 , 15  row 2: -7 , 5 , 1 ]

 

That's it!  So, what do ya think -- easiest thing on the whole site?

Here's a subtraction guy -- same idea:

[ row 1: -2  row 2: 0  row 3:  3  row 4: 6 ] - [ row 1: 7  row 2: -8  row 3: 0  row 4: 4 ]
 

                = [ row 1: -2 - 7  row 2: 0 - ( -8 )  row 3: 3 - 0  row 4: 6 - 4 ]         = [ row 1: -9  row 2: 8  row 3: 3  row 4: 2 ]

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.