How to add two foreign keys and add values in child table with reference of parent table

Solution 1:

Recommended structure (use as a base, adjust to real data):

CREATE TABLE user (
  user_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, -- unique index
  username VARCHAR(255) NOT NULL, 
  password VARBINARY(255) NOT NULL,                -- password hash, binary
  firstname VARCHAR(255) NOT NULL,
  lastname VARCHAR(255) DEFAULT NULL,
  email VARCHAR(255) DEFAULT NULL,
  address VARCHAR(255) DEFAULT NULL,
  phone BIGINT DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

CREATE TABLE question (
  question_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,  -- unique index
  question_text TEXT NOT NULL,        
  user_id INT NOT NULL,                                 -- id of the user who have asked
  CONSTRAINT user_to_question FOREIGN KEY (user_id) REFERENCES user (user_id)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

CREATE TABLE answer (
  answer_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,  -- unique index
  answer_text TEXT NOT NULL,
  question_id INT NOT NULL,                           -- what question it answers on
  user_id INT NOT NULL,                               -- id of the user who have answered
  CONSTRAINT user_to_answer FOREIGN KEY (user_id) REFERENCES user (user_id),
  CONSTRAINT question_to_answer FOREIGN KEY (question_id) REFERENCES question (question_id)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

In this scheme I do not show the restriction that the user cannot answer his own question. I consider such a restriction to be meaningless. But if you need in it nevertheless then you may add it by creating 2 triggers (BEFORE INSERT and BEFORE UPDATE) on answer table.