FORMULABANK

Depending on which side you want to find, you can use one of the following formulas:

$a^2=b^2+c^2-2bc\cdot \cos \left(A\right)$a^2=b^2+c^2−2bc⋅cos⁡(A)

$b^2=a^2+c^2-2ac\cdot \cos \left(B\right)$b^2=a^2+c^2−2ac⋅cos⁡(B)

$c^2=a^2+b^2-2ab\cdot \cos \left(C\right)$c^2=a^2+b^2−2ab⋅cos⁡(C)

The Cosine Rule:
Geometry

Since a line segment cannot be negative, we know that $a=\mathrm{=3}$