### points on elliptic curve number field

I have the following elliptic curve : y^2=x^3-3267x+45630 and generator P=[-21,324].

I want to find the general formula to add any point (r,s) to the point ((-15/2)+(27/2)*B,0) where B^2=17.

PARI seems to be pretty straightforward in giving answer. And i think the mistake I did in SAGE is that I should define r and s in number field which I'm not sure how to do it.

The Pari code is as follows ~~:
~~:

`gp > `~~elladd(E,[r,s],[(-15/2)+(27/2)~~*B,0])= [(-8*r^3 elladd(E,[r,s],[(-15/2)+(27/2)*B,0])= [(-8*r^3 + ~~(108~~*B **(108*B - *~~60)~~r^2 60)*r^2 + ~~(1458~~*B^2 **(1458*B^2 - *~~1620~~B 1620*B + ~~450)~~*r **450)*r + *~~(8~~s^2 (8*s^2 + ~~(-19683~~*B^3 **(-19683*B^3 + *~~32805~~B^2 32805*B^2 - ~~18225~~*B **18225*B + *~~3375)))/(8~~r^2 3375)))/(8*r^2 + ~~(-216~~*B **(-216*B + *~~120)~~r 120)*r + ~~(1458~~*B^2 **(1458*B^2 - *~~1620~~B 1620*B + 450)), ~~(-8~~*s*r^3 (-8*s*r^3 + ~~(4374~~*B^2 **(4374*B^2 - *~~4860~~B 4860*B + ~~1350)~~*s*r 1350)*s*r + ~~(8~~*s^3 **(8*s^3 + *~~(-39366~~B^3 (-39366*B^3 + ~~65610~~*B^2 **65610*B^2 - *~~36450~~B 36450*B + ~~6750)~~*s))/(-8*r^3 6750)*s))/(-8*r^3 + ~~(324~~*B **(324*B - *~~180)~~r^2 180)*r^2 + ~~(-4374~~*B^2 **(-4374*B^2 + *~~4860~~B 4860*B - ~~1350)~~*r **1350)*r + *~~(19683~~B^3 (19683*B^3 - ~~32805~~*B^2 **32805*B^2 + *~~18225~~B 18225*B - ~~3375))]~~3375))]