Remember when adding or subtracting matrices, the matrices must be the same size.  Corresponding elements are added or subtracted to give elements of the new matrix.

Example: 
         1  3  5
A =   5  7  8
         2  0  9

         -2  8  0
B =    3  1   1
          5  0   2

                1-2   3+8  5+0       -1  11  5
A + B  =  5+3  7+1  8+1  =    8    8  9
                2+5  0+0  9+2        7    0  11