I think your reasoning is solid and you have a correct approach.
I think the question was wanting you to use the expression at constant pressure $$V \propto T$$
This would give you the same answer because the ideal gas law $$PV = nRT$$ can be rearranged to give $$V = {nR\over P}T$$ which at constant pressure gives $$V = const.~T$$ which is equivalent to $$V \propto T$$