In $\Delta ABC$,P and Q are points on sides AB and AC respectively such that $PQ\parallel BC$.If AP=4 cm,PB=6 cm and PQ=3 cm,then BC is ____

Question

$\begin{array}{1 1}\text{7 cm}\\\text{7.5 cm}\\\text{6 cm}\\\text{6.5 cm}\end{array}$