Let $\displaystyle \textbf{u}_1$ and $\displaystyle \textbf{u}_2$ form an orthonormal basis for $\displaystyle R^2$ and let $\displaystyle \textbf{u}$ be a unit vector in $\displaystyle R^2$. If $\displaystyle \textbf{u}^T\textbf{u}_1=1/2$, determine the value of $\displaystyle |\textbf{u}^T\textbf{u}_2|$

Can someone help me begin this problem, thank you!