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Each entry in the answer matrix will work like this...

A = [ row 1: -1 , 5  row 2: 2 , -3 ] ... B = [ row 1: 0 , 9  row 2: 8 , -6  row 3: 4 , 7 ] ... use column 1 of matrix A and row 2 of matrix B and see where they intersect in the answer matrix c 

Row 2 of B and column 1 of A intersect in entry c21 of the answer matrix...

 

A = [ row 1: -1 , 5  row 2: 2 , -3 ] ... B = [ row 1: 0 , 9  row 2: 8 , -6  row 3: 4 , 7 ] ... -20 goes in entry c21 of the answer matrix c 

 

 

Multiply and add the same way:

( 8 ) ( -1 ) + ( -6 ) ( 2 ) = -20

 

Get it?  How do we fill the  c31entry?
 
A = [ row 1: -1 , 5  row 2: 2 , -3 ] ... B = [ row 1: 0 , 9  row 2: 8 , -6  row 3: 4 , 7 ] ... use column 1 of matrix A and row 3 of matrix B ... 10 goes in entry c31 of the answer matrix c 

 

 

 

( 4 ) ( -1 ) + ( 7 ) ( 2 ) = 10