www.wysiwygwebbuilder.com

My intent:

Control that someone real is using my contact form by asking to type a value my choosing

Question, what is the minimal value of C Knowing the B = 2A and C=A+B
The answer is 3.

The value to check is the answer 3.

If the answer is correct, the send button becomes visible, if not an error message is displayed.

Thank you for your help.

Just to let you that the validation method you seek, is considered "old technology" these days, and is easy for robots to figure out.

Have a look at the reCAPTCHA feature, instead.

So, how did you do it?

https://wysiwygwebbuilder.com/forms_antispam.html

https://www.wysiwygwebbuilder.com/recaptchav3.html

Here are the instructions.