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. :)
Drag the Regular Expression element onto your form.
Under the Regular Expression Properties choose custom from the RegEx field.
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 /^+$/ 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.
Name your element and setup your questions within that element. Here we have placed the question in the placeholder.
Do not forget to set your Error Message.
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. :)