Explanation

$\vec{u} = 3\hat{i} + 4\hat{j} \Rightarrow ux = 3$ and $uy = 4$; $\vec{a} = 0.4\hat{i} + 0.3\hat{j} \Rightarrow ax = 0.4$ and $ay = 0.3$ So, $V_{x} = ux + a_{x}t = 3 + 0.4 \times 10 = 3 + 4 = 7$ $v_{y} = xy + a_{y}t = 4 + 0.3 \times 10 = 4 + 3 = 7$ $\vec{V} = 7\hat{i} + 7\hat{j} \Rightarrow |\vec{V}| = \sqrt{7^{2} + 7^{2}} = 7\sqrt{2}$ units