A person travels along a straight road for the first half time with a velocity $v_1$ and the next half time with a velocity $v_2.$ The mean velocity $v$ of the person is $\left(v_1\right.$ and $v_2$ are in same direction)

The velocity-time $(v-t)$ graph for a particle moving along $x-$axis is shown in the figure. The acceleration at $t=4.5 \mathrm{s}$ is

A particle is at rest at origin at time $t=0 \mathrm{s}.$ It is given an acceleration $a=6t \mathrm{m} {\mathrm{s}}^{-2}.$ The position of the particle at $t=2 \mathrm{s}$ is