Finding the third side of an irregular triangle?

Question

lets say you have an angle in between two sides of an irregular triangle. how do you find the distance of the third side?

Answer 1

You use the cosine rule. Let a, b and c be your sides and θ be the included angle. You're trying to find length c:

c² = a² + b² - 2ac*cos θ  

Answer 2

If we have two side-lengths and the included angle, then the most direct way I know to get the third side-length is the law of cosines.