which of the following correctly shows the first step when factoring 20r^3+5r^2-28r-7 by grouping
5r^2(4r+1)+7(4r+1)
5r^2(4r+1)-7(4r-1)
5r^2(4r+1)-7(4r+1)
5r^2(4r+1)+7(4r-1)
5r^2(4r+1)+7(4r+1)
5r^2(4r+1)-7(4r-1)
5r^2(4r+1)-7(4r+1)
5r^2(4r+1)+7(4r-1)