I want the moving ball to bounce back in the processing.

First of all, I would like to point out that I cannot speak Japanese.This is my first answer.I used Google translation, so please apologize if I accidentally informed someone.:)

You need to check if the ball hit the wall (left or right).If hit, change the speed from positive (increase) to negative (decrease).

The code requires the following elements:
- If State
- or the logical operator
- Multiplication operator

Translate the above words into pseudocode:

If the x position of the ball is greater than the right wall (sketch width) or the x position of the ball is less than the left wall (0), the speed must reverse the sign (positive to negative, and vice versa).

Convert pseudocode to code:

// If the ball's x position is greater than the right or left wall (0)
if(x>width||x<0){
   // Inverted direction (speed) (+1*-1=-1, -1*-1=+1)
  velocityX=velocityX*-1;
}



velocityX=velocityX*-1; 

Write more concisely

velocityX*=-1;

Add the above to the code.

float x;
int velocityX=1;

void setup() {
  size (400,300);
  x = 0;
}

void draw() {
  background(0);

  x+ = velocityX;

  if(x>width||x<0){
    velocityX*=-1;
  }

  fill(255,255,50);
  else (x, height / 2, 20, 20);
}

Closing remarks:

- I recommend that you format the code in an orderly fashion (Ctrl+T/CMD+T): It is easy to read and, on average, spends more time reading than writing the code.
- Be sure to check the processing tutorial: bound, Boundball
- Watch Daniel Shiffman's YouTube video.In English, I hope the Closed Captioning (CC) > Automatic Translation > Japanese option will help.The translation is not perfect, but a visual explanation would make sense if it worked.