The dimensions of a matrrix is "down by across"
That is, you count how many numbers there are counting downward from
top to bottom.  That's the "down" dimension.  Then you count how many
number there are going from left to right.  That the "across" number.

In order for a matrix on the left to be multiplied by a matrix on
the right, the ACROSS number of the matrix on the left must be the
same as the DOWN number on the right. 

This is the second part of my first question:
(I put multiple periods in between the letters and numbers so that the matrices would be whole.)

Let's find the dimensions of all the given matrices: 

A: [3]   That's 1 down and 1 across, so it's a 1x1 matrix 


B:[2 9]   That's 1 down and 2 across, so it's a 1x2 matrix 


C: [2 4 -3    That's 2 down and 3 across, so it's a 2x3 matrix 
   -2 3 0] 


D:   [3 2    That's 3 down and 2 across, so it's a 3x2 matrix     
      3 2
      3 2] 


E:  [-8 -2   That's 2 down and 2 across, so it's a 2x2 matrix 
      0 0] 


F: [ 2 3 4 5 6 7 9] That's 1 down and 9 across, so it's a 1x9 matrix  


G: 
    [3
     2
     1
     0
     9
     8
     7]  That's 7 down and 1 across, so it's a 7x1 matrix 

Which matrices can be multiplied by A on the right?
(Enter the uppercase letters corresponding  to the correct answer.)

The ACROSS number of A is 1, so any matrix that can multiply by A on
the right must have a DOWN number of 1.  Only A, B and F have DOWN numbers
of 1. 

Now be sure you understand why those are the correct answers.  Don't just enter
them in the computer and forget it.

Edwin