This may seem hard at first because you are getting into uncharted territory with regular expressions. But once you see the process it really is not that tough. :) 

Step 1: 

Drag the Regular Expression element onto your form. 

Step 2: 

Under the Regular Expression Properties choose custom from the RegEx field.

Step 3:

 Here is where you need to set your question/answer. In this example we are using 2*2 which equals 4. So our regular expression would read /^[4]+$/ This tells the form that the only correct answer that can be entered into this field is 4. 

 This example only works with single digit answers.

Step 4:

Name your element and setup your questions within that element. Here we have placed the question in the placeholder. 

Step 5: 

Do not forget to set your Error Message. 

Step 6:

Test your form! You are done. :) In the end your form should look like this: Please note that the below form is an image. You can not fill it out. :)