Sir, can you please explain why 2i isn't acceptable as an answer for that question (Q. 10 Ex 3.3)?
2i is a correct answer!
The answer is in the form a + bi
If b = +2i then we would work out that a = -1i.
So then a + bi = -1i + (2i)i = -1i - 2 = -2 - 1i
Likewise if b = -2i and a = 1i then a + bi = 1i + (-2i)i = 2 - i
This a repeat of the answer we get when b = +1 or -1.
The logic for choosing b = +1 or -1 instead of +2i or -2i is that in the number a + bi the b part is a real number.
The imaginary number is bi not b!
Therefore it makes sense to choose b = +1 or -1 over -2i or +2i because we want b to be real and bi to be imaginary.