We Multiply first row by first column in the other matrice
Let the first matrice be A
and the second be B
A*B=C (Another matrice)
Notice that A is a 3x3 matrice (Because it has 3 rows and 3 columns)
To multiply it by any other matrice it must be 3*x(Where x could be any number >1 of course)
The endings must be the same
for example, 4x4 matrice can only be multiplied by 4*x matrice
Back to the origional question now,
A is 3x3 matrice
B is 3x2 matrice
Both could be multiplied.
A row is the horizontal arrangement of numbers
e.g: 1 3 5 row 1
2 5 7 row 2 etc..
The column is the vertical arrangement of numbers
col.1 col.2 col.3 and so on.
1 2 5
5 7 4
When we multiply 2 matrices we multiply First row by the column in the second matrice
First row of A * First column of B
so -1*2 + 2*7 + 0*1 (Notice we add them together to just represent 1 number that is written at row 1, column 1) and so on till we do the 3 rows then switch column and redo it :). hope that helps.