So what did i do wrong?
In here, you're mod-ing 7 with p instead of (x
3 + 7) with p:
(55066263022277343669578718895168534326250603453777594175500187360389116729240)³ + 7 mod 115792089237316195423570985008687907853269984665640564039457584007908834671663
If you mod (x
3 + 7) with p, you will, indeed, get:
32748224938747404814623910738487752935528512903530129802856995983256684603122