How can I multiply two matrices of different dimensions?

I need to multiply a 3x9 and 2x2 matrix with my calculator but it keeps saying
"ERR: DIM MISMATCH"

Question

How can I multiply two matrices of different dimensions?

I need to multiply a 3x9 and 2x2 matrix with my calculator but it keeps saying 
"ERR: DIM MISMATCH"

anonymous · Answer

The reason there is a dimension mismatch is because when you can not multiply those, the inner numbers of each matrix havr to have the same value  eg.. a 3X 5 can multiply a 5X7 because the two inner numbers are the same  the answer is a 3 X 7.. if you were asked to multiply a 3X7 times a 3X5 you could not multiply these because the two inner numbers  7 and 3 are not alike.