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=\text{13}\centerdot \text{tan 27}^{\circ }.

NOTE: Using a calculator, we can work out what \displaystyle x\text{ } should be:

\displaystyle x=\text{13}\centerdot \text{tan 27}^{\circ }\approx 6.62