main.go

package main

import (
	"encoding/hex"
	"fmt"

	"golang.org/x/crypto/curve25519"
)

func main() {
	a, _ := hex.DecodeString("0300000000000000000000000000000000000000000000000000000000000000")
	b, _ := hex.DecodeString("0900000000000000000000000000000000000000000000000000000000000000")
	var ax, bx, dx [32]byte
	copy(ax[:], a)
	copy(bx[:], b)
	curve25519.ScalarMult(&dx, &ax, &bx)
	fmt.Printf("%x\n", dx)
}

Result:

$ go run main.go
2fe57da347cd62431528daac5fbb290730fff684afc4cfc2ed90995f58cb3b74