Here is a hint:
Change z to a+bi. Since the magnitude is the square root of a^2+b^2, solve for that.
To make myself more clear, let me show you how to do the left side, and then you can to the right side.
Once we have changed z to a+bi, we can subtract 3 from a. This will give us (a-3)+bi.
Now, squaring a and b, we get
a^2-6a+9+b^2 for the inside of the square root. Therefore, the left side is \(\sqrt {a^2-6a+9+b^2}\).
Now, try to do the right side, and solve the equation.