An athlete, initially at rest, takes 2s to reach his maximum speed...

The final speed of the athlete, $v = 36km{h}^{- 1} = 36 \times \dfrac{5}{18} = 10m{s}^{- 1}$

initial speed, $u = 0$

time taken $t = 2s$

for average acceleration, $a$

$v = u + at$

$a = \dfrac{v - u}{t} = \dfrac{10 - 0}{2} = 5m{s}^{- 2}$