Your question is too general... there's a huge number of signals that would fit into that spectral mask. Post more specific questions once you get started.
So far what you know (from the picture):
0. Signal must be centered at 21MHz (common IF frequency)
1. 3dB bandwidth must be 5.25MHz
I'm not sure how much you know or don't know about signals so I'll leave it at that. You should be able to find basic Matlab tutorials online. Here's a lab I found with a quick search:
http://www.emba.uvm.edu/~gmirchan/classes/EE275/2012/Matlab/Lab1.pdf[
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