A block of mass 1.2 kg is held by two springs of 200 N/m and 300 N/m…

(a) $k = 200\ \text{N/m} + 300\ \text{N/m} = 500\ \text{N/m}$, so $T = 2\pi\sqrt{\dfrac{1.2\ \text{kg}}{500\ \text{N/m}}} = 0.308\ \text{s}$.

(b) $k = \dfrac{200\ \text{N/m} \times 300\ \text{N/m}}{500\ \text{N/m}} = 120\ \text{N/m}$, so $T = 2\pi\sqrt{\dfrac{1.2\ \text{kg}}{120\ \text{N/m}}} = 0.628\ \text{s}$.