I would have interpreted the problem as Aladin did; it asks for the sum of the diagonal of the product of the two matrices. You could do that in two steps (calculate the product then the sum), or all at once:
A
B
C
D
E
F
G
H
I
1
7
4
0
-2
12
-10
G1:H2: {=MMULT(A1:B2, D1:E2)}
2
1
1
3
1
3
-1
3
4
11
H4: =G1+H2
5
6
11
H6: =SUMPRODUCT(MMULT(A1:B2, D1:E2), {1,0;0,1})
I interpret a11 and a12 as the diagonal elements of the matrix {a11, a12; a21, a22}
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