Can someone please explain me with a proper answer, I didn't understand the questionThank you!

Dear student

Step 1 assume a complex number where imaginary part cant be zero
step 2 you can expand the term using binomial expansion
Step 3 collect imaginary terms from the above expansion

Step 4 is ratio of { terms of step 3}/{ b^5} ( denominator can be read as (im z)^5

Step 5 is just simplification and assuming a/b to be x just have quadratic expression

Step 6 is further simplification,
Now, minimum value of y can be obtained by taking minimum value of 5(x-1)^2 ie 0

Therefore ymin. = -4

Regards