When you multiply a (2x3) matrix by a (3x1) matrix, you 
first find out what kind of matrix you're going to get
by this method:

(x) by (x)

Cross out the inner dimensions, which must be the same:

(x) by (x)

and you get a 

x matrix.

So we write this with a {2x1) matrix with
question marks for elements



To get what goes in place of the 1st question mark,

Multiply each element in the 1st row of the (2x3) matrix
by the corresponding element in the (3x1), then add,
like this:

, so we have



-------------------

To get what goes in place of the 2nd question mark,

Multiply each element in the 2nd row of the (2x3) matrix
by the corresponding element in the (3x1), then add,
like this:

, so we have



-------------------

Edwin