For relevant on a select_multiple you will want to use:
selected(${question_name}, 'answer_name')
(be sure to wrap the answer part in quotes)
And not:
${question_name} = 'answer_name'
Also, welcome to the ODK forum, @ODK_Ekomer! We're glad you're here. When you get a chance, please introduce yourself on this forum thread. I'd also encourage you to add a picture as your avatar because it helps build community!