How can I align the edges of a cut cylinder with another cylinder? So I have two cylinders, one is cut and one is not. 

Here is the misaligned cylinder (That is cut at 45 degrees): 



Is is possible to align those edges and make them straight again with the cylinder behind it? 

Edit: 

Kind of like this, where the green lines represent how I want the selected objects edges to be aligned with.
Select the vertices that you want to flatten, then scale it along the axis you want to flatten by 0. 

In this case, scale along the y-axis by 0.