I learned from @JayChow! I started by doing the CL scavenger hunt and then just kept learning by making activities. There are webinars that @JayChow has hosted that are archived somewhere in this forum.
For your question, don’t forget to mark the sketch as readOnly: true. For the input, try this:
correct = simpleFunction(this.latex, "a", "b").evaluateAt(5,5)=5 and countNumberUsage(this.latex,2)=1 and countNumberUsage(this.latex,3)=1 and countNumberUsage(this.latex)=2
For the evaluateAt, I just chose to sub 5 for a and b, which happens to evaluate to 5. You can pick other numbers too. I also look for one 3, one 2 (countNumberUsage only looks for positive values), and two numbers total.