Suppose the correct answer to a question is the inequality

ad - bc != 0.

Is there a good way to check such an answer in WW? My current solution is to tell the students that their answer must have the form

[some expression in a, b, c, d] [= or != (popup)] [value].

The answer checker I'm currently using allows for two possibilities based on how I expect students to solve the problem: it will accept in the first blank an algebraic expression that is equivalent to ad - bc or equivalent to d - bc/a (a is assumed nonzero in this problem). But of course 3(ad - bc) would be a valid expression as well ... it is unlikely that students would come up with this, but it's technically correct, so ...

Any ideas would be appreciated!