Hypothesis Tests · Superchallenge problem

Six plants are split at random: fertilizer group heights $7, 9, 12$; control group heights $4, 5, 8$. Under $H_0$ (fertilizer does nothing), every way of choosing which $3$ of the $6$ heights form the "fertilizer" group is equally likely. What is the probability that the fertilizer group’s total is at least the observed $28$?