How to plot the centroid and bounding box of an image?

HI i'm trying to do solve a problem where i've to find plot the centroid and bouding box of the image and then i've to plot it. So far i've been able to do this and now i don't understand how do i plot this.

a = imread('iPAD2_C23_EX05.JPG');

c = imbinarize(rgb2gray(a));

[L,n] = bwlabel(true(size(c)));

z = regionprops(b, 'Centroid','BoundingBox');

x = z(k).Centroid(1);

y = z(k).Centroid(2);

text(x,y,' \leftarrow G','FontSize',18,'color','r')

But, i'm getting only a blank plot. I also want to plot the bounding box but there's no example availabe online.