How can i get RectTransform width to set it to my GridLayoutGroup's Cell Size X?

how to get RectTransform width properties and set it to cell size X if my hierarchy looks like below picture?

here's RectTransform anchors presets of the objects. [Panel - Content] Anchors min(0,1), max(1,1) [Panel - Scroll] Anchors min(0,0), max(1,1)

I've tried the following - UIPanelRectTransform.rect.width but it gives me -10 - UIPanelRectTransform.rect.x gives me 5 - UIPanelRectTransform.sizeDelta.x it gives me 10

what i'm trying to do is to set UIPanel's GridLayout CellSize X that is equal to UIPanel's RectTransform Width, how can i get it? please help me

thank you.