That would be correct...it was a constant.
If is a function of x, then
If isn't a constant, but rather a different variable, then define . Thus, the [partial] derivative with respect to x would be .**
Does this help?
** If you don't understand this part, that's ok. I'm just trying to cover all possibilities. Pick the one that best answers your question based off of where you're at with respect to differentiation.