The topological order for the given hasse diagram is
$a\leq \_\_ \leq \_\_ \leq d \leq \_\_ \leq \_\_ \leq \_\_ \leq h.$
The first two blanks can be filled with $b$ and $c$ in any order, so $2!$ ways.
Next three blanks can be filled with $e,f$ and $g$ in any order, so $3!$ ways.
So, total orderings possible $= 2! * 3! = 12.$