ANSWER
[tex]12m {s}^{ - 1} [/tex]
EXPLANATION
The acceleration of the runner is given by,
[tex]acceleration = \frac{velocity}{time } [/tex]
If the runner accelerates at the rate of
[tex]6m {s}^{2} [/tex]
for 2 seconds, then,
[tex]6 = \frac{velocity}{2} [/tex]
We multiply through by 2 to get,
[tex]12 = velocity[/tex]
Therefore the runner is moving at a velocity of
[tex]12 \: m {s}^{ - 1} [/tex]
