Hey Andrea,
There’s no way to make one side of a div convex like this.
You have to use another method. I can see two quite evident:
- make an image of the curve, with on the bottom and transparent on the top and stick it to the bottom of the upper section, to make a smooth transition to the second section.
or
- have a div set to absolute on the background of the second section. make it white and make it super large, larger than your page (use overflow hidden on the parent). Play with the corner radius with values in % to achieve the curve.