Now we required to use angle functions in a right triangle. The cosine of [tex]\beta[/tex] is equal to the relation of side [tex]a[/tex] and hypotenuse [tex]c[/tex].
So we know the length of hypotenuse and length of side [tex]a[/tex] therefore side [tex]b[/tex] can be calculated using Pythagorean theorem: [tex]c^2=a^2+b^2[/tex].
Solve for side [tex]b[/tex].
[tex]c^2=a^2+b^2[/tex]
[tex]b=\sqrt{c^2-a^2}[/tex]
[tex]b=\sqrt{13.7^2-9^2}\approx\boxed{9.7}[/tex]
So there you go. If you have any questions feel free to ask.