$\begin{array}{l}
\,\,\,\,\,\,\,\,\,\,\,\,\,Here,\\
Initial\,velocity,\,\overline u  = 3\hat i + 4\hat j\\
Acceleration,\,\overline a  = 0.4\hat i + 0.3\hat j\\
Time\,,\,t = 10\,s\\
Let\,\overline v \,be\,velocity\,of\,a\,particle\,\\
after\,10\,s.
\end{array}$

$\begin{array}{l}
{\rm{Using,}}\,\overline v \, = \overline u  + \,\overline a t\\
\therefore \,\,\,\overline v  = \left( {3\hat i + 4\hat j} \right) + \left( {0.4\hat i + 0.3\hat j} \right)\left( {10} \right)\\
\,\,\,\,\,\,\,\,\, = \,3\hat i + 4\hat j + 4\hat i + 3\hat j = 7\hat i + 7\hat j\\
Speed\,of\,the\,particle\,after\,10\,s = \left| {\overline v } \right|\\
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \sqrt {{{\left( 7 \right)}^2} + {{\left( 7 \right)}^2}}  = 7\sqrt 2 \,units
\end{array}$