a) Water - $H_2O$.

A water molecule has 2 hydrogen atoms ($H$) and 1 oxygen atom ($O$) so the molecular weight is

$2 \times \var{h} + \var{o} = \var{2 * h + o}$.

b) Methane - $CH_4$.

A methane molecule has 1 carbon atom ($C$) and 4 hydrogen atoms ($H$) so the molecular weight is

$\var{c} + 4 \times \var{h} = \var{c + 4 * h}$.

c) Glucose - $C_6H_{12}O_6$.

A glucose molecule has 6 carbon atoms ($C$), 12 hydrogen atoms ($H$) and 6 oxygen atoms ($O$) so the molecular weight is 

$6 \times \var{c} + 12 \times \var{h} + 6 \times \var{o} = \var{6 * c + 12 * h + 6 * o}$.

d) Calcium phospate - $Ca_3(PO_4)_2$.

A calcium phosphate molecule has 3 calcium atoms ($Ca$) and 2 phosphate ions ($PO_4$). We don't need to worry too much about what an ion is here, we just need to know that $(PO_4)_2$ means there are 2 lots of $PO_4$ which means a total of 2 phosporus atoms ($P$) and $4 \times 2 = 8$ oxygen atoms ($O$). Therefore, the molecular weight is 

$3 \times \var{ca} + 2 \times \var{p} + 8 \times \var{o} = \var{3 * ca + 2 * p + 8 * o}$.