I don't get the answer required. Where am I wrong? Help would be appreciated. :)

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Q:

A line has equation $\displaystyle \bold r = 3 \bold i - 5 \bold j + 2 \bold k + \lambda (2 \bold i -4 \bold j + \bold k)$ and a plane has equation $\displaystyle \bold r . (3 \bold i - \bold j - 5 \bold k) = 1$. Find the acute angle between the line and the plane.

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My Method:

$\displaystyle \bold n . \bold n = | \bold n | | \bold b | \cos \phi$

$\displaystyle \begin{pmatrix} 3 \\ -1 \\ 5 \end{pmatrix} . \begin{pmatrix} 2 \\ -4 \\ 1 \end{pmatrix} = (\sqrt {35}) (\sqrt {21}) \cos \phi$

$\displaystyle \therefore \cos \phi = \frac {15}{7 \sqrt {15}} = 56.4^{\circ}$

$\displaystyle \implies \theta = 33.6^{\circ} (90^{\circ} - 56.4^{\circ})$.

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The Correct Answer:

$\displaystyle 10.6^{\circ}$.