The matrices in the form [a11a21a12a22],aij∈{0,1,2},a11=a12 are [00/1/20/1/20],[10/1/20/1/21],[20/1/20/1/22] At any place, 0/1/2 means 0,1 or 2 will be the element at that place. Hence there are total 27=3×3+3×3+3×3 ) matrices of the above form. Out of which the matrices which are singular are [000/1/20],[01/200],[1111],[2222] Hence there are total 7(=3+2+1+1) singular matrices. Therefore number of all non-singular matrices in the given form =27−7=20