Answer
$c\left[ \begin{array}{ c c } b \\ -{a} \end{array} \right]$
Work Step by Step
Given u=(x,y),v=(a,b)
vector u and v are orthogonal if there dot product is zero.
$u.v=0$
$x\cdot a+y \cdot b=0$
$ax+by=0$
$y=-\frac{ax}{b}$
So the vector orthogonal to u is:
$ \left[ \begin{array}{ c c } x \\ -\frac{a}{b}x \end{array} \right]=\frac{x}{b}\left[ \begin{array}{ c c } b \\ -{a} \end{array} \right]=c\left[ \begin{array}{ c c } b \\ -{a} \end{array} \right]$
