[list=1][*]Add a point [b]O=(0,0,0)[/b] and points [b]A[/b] and [b]B[/b][br][/*][*]Create the Vector between Two Points between [b]O[/b] and [b]A[/b] and between [b]O[/b] and[b] B [/b]and rename these as [b]a[/b] and [b]b[/b][br][/*][*]Add the line: [b]Line[A,b][/b][br][/*][*]Add a point on the line and rename it [b]P[/b][br][/*][*]Add the vector between [b]O [/b]and [b]P[/b]  and rename it [b]r[/b][br][/*][/list]