\begin{array}{l}
\mathrm{u}=90 \mathrm{~km} / \mathrm{hr}=\frac{90 \times 1000 \mathrm{~m}}{1 \times 3600 \mathrm{~s}}=25 \mathrm{~m} / \mathrm{s} \\
\mathrm{v}=0, \mathrm{t}=10 \mathrm{~s}
\end{array}
\(a=\frac{v-u}{t}=\left(\frac{0-25}{10}\right) \mathrm{m} / \mathrm{s}^2=-2.5 \mathrm{~m} / \mathrm{s}^2\)