Ihave a 1x2 matrix times a 2x1 matrix times a 1x2 matrix. how do i multiply three matrices

First you make sure you CAN multiply them (which you can in this case, since the inner dimensions 1 and 1 match). 

Your outer dimensions will be your new dimensions (in this case your new matrix will be 2x2) 

Then you just multiply 2x4, and put in the upper left side of your new [2x2] matrix. Then 2x2, 3x4, and 3x2.

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She should use 12/3 to measure