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 ${\mathrm{X}}_{\mathrm{P}}\left(\mathrm{t}\right)=\mathrm{\alpha t}+{\mathrm{\beta t}}^2$ and ${\mathrm{X}}_{\mathrm{Q}}\left(\mathrm{t}\right)=\mathrm{ft}-{\mathrm{t}}^2$. At what time, both the buses have same velocity?