Convert speeds to m/s
[tex]\\ \rm\longmapsto u=20\times\dfrac{5}{18}=5.05m/s[/tex]
[tex]\\ \rm\longmapsto v=70\times \dfrac{5}{18}=19.4m/s[/tex]
[tex]\boxed{\sf Acceleration=\dfrac{v-u}{t}}[/tex]
[tex]\\ \rm\longmapsto Acceleration=\dfrac{19.4-5.05}{10}[/tex]
[tex]\\ \rm\longmapsto Acceleration=\dfrac{14.3}{10}[/tex]
[tex]\\ \rm\longmapsto Acceleration=143m/s^2[/tex]