How to find the parameters (a, h) of an elliptic curve?

Question

go

I'm using Go, and there, under the elliptic.P256() curve from crypto/elliptic , it means secp256r1 The recommended
parameters for this elliptic curve can be checked by comparing with
https://www.secg.org/sec2-v2.pdf#page=13&zoom=100 ,0,884
But CurveParams does not contain all parameters

type CurveParams struct {
  P       *big.Int // the order of the underlying field
  N       *big.Int // the order of the base point
  B       *big.Int // the constant of the curve equation
  Gx, Gy  *big.Int // (x,y) of the base point
  BitSize int      // the size of the underlying field
  Name    string   // the canonical name of the curve
}

Not enough A and H , as I understand it, they should be calculated from the available ones.
Since the curve formula is y^2 = x^3 + ax + b
Tried to solve it like this a = (y^2 - x^3 - b)/x

B, _ := new(big.Int).SetString("5ac635d8aa3a93e7b3ebbd55769886bc651d06b0cc53b0f63bce3c3e27d2604b", 16)
Gx, _ := new(big.Int).SetString("6b17d1f2e12c4247f8bce6e563a440f277037d812deb33a0f4a13945d898c296", 16)
Gy, _ := new(big.Int).SetString("4fe342e2fe1a7f9b8ee7eb4a7c0f9e162bce33576b315ececbb6406837bf51f5", 16)

x3:=new(big.Int).Mul(Gx, Gx)
x3.Mul(x3, Gx)
Gy.Mul(Gy, Gy)
Gy.Sub(Gy, x3)
Gy.Sub(Gy, B)
a := Gy.Div(Gy, Gx)

aR,_ := new(big.Int).SetString("FFFFFFFF00000001000000000000000000000000FFFFFFFFFFFFFFFFFFFFFFFC", 16)

fmt.Print(aR.Cmp(a)==0, aR, a)

But the result doesn't match.