Answer
Since HOBBIT is wildtype and monopteros is mutant, no proper root forms since monopteros, acting first, is not active and thus does not allow for a normal root to form.
Work Step by Step
Realize that in double mutants, if one gene acts before the other and is a mutant, only the mutant phenotype is seen since the second-acting gene needs the first gene to have had its proper effect before it can, in turn, act.