Two cars P and Q start from a point at the same time in a straight line and their positions are represented by ${\mathrm{x}}_{\text{P}}\left(\text{t}\right)=(\text{at}+{\text{bt}}^2)$and ${\mathrm{x}}_{\text{Q}}\left(\text{t}\right)=(\text{ft}-{\text{t}}^2)$. At what time do the car A has double velocity of B ?