We want to find a vector $\mathbf{v}=x\mathbf{i}+y\mathbf{j}$ that is orthogonal to $\mathbf{u}$. This means that the dot product of $\mathbf{u}$ and $\mathbf{v}$ should be zero. In other words, we want to solve the equation
$$
\mathbf{u} \cdot \mathbf{v} =
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