Using the tangent line at that point.<p><a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Elliptic_curve_point_multiplication#Point_doubling" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Elliptic_curve_point_multiplic...</a>