if i want to find the complex number of say:
|z|=3 rt(7) and Re(z)=0 ....point of intersection
then id get to y^2= 3 rt(7) and would i take the negative root and just make into a complex number so 3rt(7) i
??
Geometrically it is seen that $\displaystyle |z|=3 \sqrt{7}$ is a circle of radius $\displaystyle 3 \sqrt{7}$ with centre at the origin. Re(z) = 0 is the imaginary axis. Clearly then the points of intersection are $\displaystyle (0, 3 \sqrt{7})$ and $\displaystyle (0, - 3 \sqrt{7})$.
By the way, $\displaystyle |iy| = 3 \sqrt{7} \Rightarrow |i| |y| = 3 \sqrt{7} \Rightarrow |y| = 3 \sqrt{7} \Rightarrow y = \pm 3 \sqrt{7}$.
Again, you've misunderstood how to get the magnitude of a complex number.