I want to be able to have my students work through questions in a Mathematica notebook and then run a piece of code that compares their solutions to what I want.

I thought along these lines:

NotebookLocate["Q18a"]; SelectionEvaluate[EvaluationNotebook[]];
If[% == 47/12 - 3/2(x + 25/6)^2,"CORRECT", "WRONG"]

Q18a is a Cell Tag for the cell the answer is in. The cell gets evaluated and then the last evaluation (the cell) is compared to the answer. But I keep getting responses like this as the output:

If[Null==47/12 - 3/2(x + 25/6)^2,CORRECT,WRONG]

Anyone able to fix or suggest a better way?

Thanks in advance.