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HTML/CSS Assessment min screen length

Hello,
I’ve been trying to get the form to be the width of the screen when the the screen is under 600px, but it seems to be less than that when i check it out. When the screen width is 599px the form width is 583px, should the form width be exactly the same as the width of the screen in pixels? The from looks like it takes up the whole screen, but the pixels are off. Does anyone have any suggestions from my code?
Thank you in advance!

This is the photo of the screen a 599px and the form at 583px.