A little help with trig please.

Something like this what you mean?

	Graphics 800,600
	SetBuffer BackBuffer()
	
	; Set the two vertices defining the 'start' line.
	start_x# = 100
	start_y# = 300
	end_x# = 120
	end_y# = 180
	
	delta_x = end_x - start_x
	delta_y = end_y - start_y

	ang# = ATan2(delta_y,delta_x)
	side_len# = Sqr(delta_x*delta_x + delta_y*delta_y)
		
	While Not KeyHit(1)
	
		Cls

		Locate 0,0
		num_sides = Input("Enter number of sides: ")

		; Draw 'start' line in red.
		Color 255,0,0
		Line start_x,start_y,end_x,end_y
		
		; Draw remaining poly sides in white.
		Color 255,255,255
		x1 = end_x
		y1 = end_y
		d_ang# = 360.0 / num_sides
		For n = 1 To num_sides-1
			x2 = x1 + (Cos((d_ang*n) + ang) * side_len)
			y2 = y1 + (Sin((d_ang*n) + ang) * side_len)
			
			Line x1,y1,x2,y2
			
			x1 = x2
			y1 = y2
		Next
	
		VWait : Flip False
		
	Wend
	
	End