Answer
$\vec{u}=3\vec{i}$
Work Step by Step
$\vec{u}=\langle3,0\rangle$
To write in terms of $\vec{i}$ and $\vec{j}$, just write the first component of $\vec{u}$ times $\vec{i}$ plus the second component of $\vec{u}$ times $\vec{j}$:
$\vec{u}=\langle3,0\rangle=3\vec{i}+0\vec{j}=3\vec{i}$
