Answer
This synthesis is done via dehydrohalogenation of bromocyclohexane obtained in part $(a)$. The mechanism is shown in the attached image.
Work Step by Step
For synthesising cyclohexene, we use the $bromocyclohexane$ formed in part (a), and heat it in the presence of a strong base such as $KOH$ or $KO-t-But$. This is a dehydrohalogenation reaction which leads to the loss of $HBr$ and forms $cyclohexene$.