jmc

We notice that in the units column, it's impossible for $△+2=1_6$. So, it must be the case that $△+2=11_6=7$. That would mean $△=7-2=\boxed{5}$. We can check that our answer works by plugging our value for triangle into the original problem: \begin{array}{c@{}c@{\;}c@{}c@{}c@{}c} & & 3 & 2 & 1 & 5_6\\ & & & 5 & 4 & 0_6\\ &+ & & & 5 & 2_6\\ \cline{2-6} & & 4 & 2 & 5 & 1_6.\\ \end{array}The addition problem still works, so our answer is correct.