Slider Image doesn't stretch past 1140 width

I'm wondering if anyone could help me. I extended the width of my homepage, and now my slider image has white space on its left and right sides. I uploaded an image to the slider that was 1200 x 400 px but it simply will not show anything past 1140 px width. What code do I need to add to Custom CSS so that I can make the slider stretch across to 1200 px or greater? Thank you!:)