Expanding the left side of the equation:
(1 + 2)(2 + 1) + blank
= 3(3) + blank
= 9 + blank
Expanding the right side of the equation:
5(2 + i)
= 10 + 5i
Since the left side of the equation (9 + blank) is equal to the right side (10 + 5i), we can deduce that the missing term should be equal to 1i.
Therefore, the missing term in the equation is "1i".