Two buses \( P \) and \( Q \) start from a point at the same time and move in a straight line and their positions are represented by \( X_P(t)=\alpha t+\beta t^2 \) and \( X_Q(t)= f t - t^2 \). At what time, both the buses have same velocity?
\( \dfrac{\alpha - f}{1+\beta} \)
\( \dfrac{\alpha + f}{2(\beta - 1)} \)
\( \dfrac{\alpha + f}{2(1+\beta)} \)
\( \dfrac{f - \alpha}{2(1+\beta)} \)