From Förberedande kurs i matematik 1

The definition of the tangent states that

\displaystyle \tan u=\frac{\text{opposite}}{\text{adjacent}}

In our case, this means that

\displaystyle \tan 27^{\circ} = \frac{x}{13}

which gives \displaystyle x = 13\cdot \tan 27^{\circ}\,.

Note: Using a calculator, we can work out what x should be,

\displaystyle x = 13\cdot\tan 27^{\circ} \approx 6\textrm{.}62\,\textrm{.}