Here, the point of suspension has an acceleration. $\overrightarrow{\text{a}}$ $=g\sin \theta$ (down the plane). Further, $\overrightarrow{g}$ can be resolved into two components g sin  θ (along the plane) and g cos  θ (perpendicular to the plane).


${\overrightarrow{g}}_{\text{eff}}=\overrightarrow{g}-\overrightarrow{\text{a}}$

$=g\cos \theta$ (perpendicular to plane)

$\mathit{\text{T}}=2\text{π}\sqrt{\frac{l}{\left|{\overrightarrow{g}}_{\text{eff}}\right|}}$

    $=2\text{π}\sqrt{\frac{l}{\mathit{\text{g}}\text{cos}\text{θ}}}$