Convert slope to bearing?

I calculate the slope of a line with the following equation:

(in VB)

slope=(y2-y1)/(x2-x1)


I would like to convert this slope to a bearing (i.e. the angle from the North), it seems that I would get the angle with

Atn(slope)*180/PI

however, it seems it's not the bearing...
and how do handle those slope with negative value?

Thanks very much!!!

Hi, I'm using this one:

VB Code:

Public Sub bearing(X1, Y1, X2, Y2, peilung)
'Input X1, Y1, X2, Y2
'Output peilung (0-360)
'calculates Bearing from X1,Y1 to X2,Y2 using a 360 degree-system
Const PI = 3.141592654
Dim dx As Single
Dim dy As Single
dx = X2 - X1
dy = Y2 - Y1
If dy <> 0 Then
        peilung = (Atn(dx / dy) * 180 / PI)
        If X2 > X1 Then
                If Y2 > Y1 Then
                        peilung = peilung
                Else
                        peilung = 180 + peilung
                End If
        Else
                If Y2 > Y1 Then
                        peilung = 360 + peilung
                Else
                        peilung = 180 + peilung
                End If
        End If
Else
        If X1 > X2 Then
                peilung = 270
        Else
                peilung = 90
        End If
End If
End Sub