The same force makes both light springs stretch.
(a) The hanging mass moves down by
$$x=x_{1}+x_{2}=\dfrac{mg}{k_{1}}+\dfrac{mg}{k_{2}}=mg\left ( \dfrac{1}{k_{1}}+\dfrac{1}{k_{2}} \right )$$
$$=(1.5kg)(9.8m/s^{2})\left ( \dfrac{1}{1200N/m}+\dfrac{1}{1800N/m} \right )$$
$$=2.04\times 10^{-2}m$$
(b) We define the effective spring constant as
$$k=\dfrac{F}{x}=\dfrac{mg}{mg(1/k_{1}+1/k_{2})}=\left ( \dfrac{1}{k_{1}}+\dfrac{1}{k_{2}} \right )^{-1}$$
$$=\left ( \dfrac{1}{1200N/m}+\dfrac{1}{1800N/m} \right )=720N/m$$