<s>[INST] Describe the set of points $z$ in the complex plane that satisfies the following equation. $|2z−i|=4$
I have tried solving it multiple times but I cannot. Here are my steps. I hope somebody can catch my mistakes. 
$|2z−i|=4$
$|2(a+bi)−i|=4$
$|(2a+2bi)−i|=4$
$|(2a+2bi)^2+(0\cdot-i)^2|=4$    (because every number is a complex number)
$|2a^2-2b^2+4\cdot a\cdot bi+1|=4$
$\sqrt{2a^2-2b^2+4\cdot a\cdot bi+1}=4$
I can’t solve this square root. Where did i go wrong . According to the book the answer is
circle, center = $i/2$, radius= $2$
I have solved other problems of it sorts but this one i couldnt.
Thanks for reading and hopefully for answering. 
 [/INST] At the end of step 3) you have
$|2a + 2bi - i|=4$. Try to combine the $i$ terms:
$|2a + (2b-1)i| = 4$.  What you have here is the complex number $w = 2z -i = 2a + (2b-1)i$ so that $Re(w) = 2a (= 2Re(z))$ and $Im(w) = 2b -1(= 2Re(z) - 1)$.
Now you can do the  formula that $|a+bi| = \sqrt{a^2 + b^2}$ or in this case
$|2a + (2b-1)i| = \sqrt{(2a)^2 + (2b-1)^2} = 4$.
So $4a^2 + 4b^2 - 4b +1 = 16$ which I suppose you could solve
$(2b-1)^2 = 16-4a^2 = 4(4-a^2)$
$2b - 1 = \pm 2\sqrt{2-a^2}$
$b = \pm \sqrt{2-a^2}+\frac 12$ so $z$ is any complex number $j + ki$ where $k = \pm \sqrt{2-j^2} + \frac 12$.
But what does that mean?
Well, bear in mind a complex number can be viewed as a point in a plane.
So if $w = 2z-i$ then $w = x+yi$ is a point in the plane.  We are given that $|w| = 4$ so that means that $w$, if viewed as the point $(x,y)$ is a distance of $4$ from the origin or that $(x,y)$ in in a circle of radius $4$ around the origin (and $x^2 + y^2 = 4^2$.  
But if all possible $w$ form a circle of radius $4$ around the origin.  Then $2z = w+i$ are the points that form of the circle around the point $(0,1) = 0 + 1i = i$ with radius $4$.  
So $2z$ are all the points $x+ yi$ where $x^2 + (y-1)^2 = 4^2$
And so $z$ is all the points where the $w+ u i$ where $w,u$ are half the values of those points in the circle. So $z = w+ui$ where $(2w)^2 + (2u-1)^2 = 4^2$ or
$w^2 + (u-\frac 12)^2 = 4 = 2^2$ or 
in other words $z$ is all the point in a circle of radius $2$ around the point $\frac 12i = 0 + \frac 12 i= (0, \frac 12)$.
It is all $z = w+ui$ where $w^2 +(u-\frac 12)i = 2^2$
</s>