Thank you for the opportunity to help you with your question!
Total number of striped shirts sold = 19
Unsold shirts have only printed and plain colored shirts. It means, in total Peter's shop had only 19 striped shirts (and all are sold now)
Total number of printed shirts from the remaining = 58
Total number of plain colored shirts remaining = 81,
Now, this 81 can also be the number of plain color shirts before the sales. If that is the case, then there is some information missing in the question (like the proportion of striped, printed and plain shirts in the sold ones).
Otherwise, plain shirts are more than printed shirts by (81-58) 23.
Please let me know if you need any clarification. I'm always happy to answer your questions. Kindly best this answer if it has helped you.