Vector3.Project

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class App : MonoBehaviour
{
    public GameObject cube;
    // Start is called before the first frame update
    void Start()
    {
        
    }

    // Update is called once per frame
    void Update()
    {
        
    }

    void OnDrawGizmos()
    {
        Vector3 a = this.cube.transform.position;// new Vector3(-1.0f, 0, 0);
        Vector3 b = new Vector3(7.0f, 0, 0);
        Vector3 c = new Vector3(0, 0, 7);

        Gizmos.color = Color.gray * 0.3f;
        Gizmos.DrawLine(c, a);
        Gizmos.DrawLine(c, b);

        Gizmos.color = Color.green * 0.5f;
        Gizmos.DrawLine(a, b);

        Gizmos.color = Color.magenta;
        Gizmos.DrawLine(a, Vector3.Project(c - a, cube.transform.forward)+a);
    }
}