1. The problem statement, all variables and given/known data In triabgle ABC, AB = 2x cm, AC = x cm, BC = 21 cm and angle BAC = 120 degrees. Calculate the value of x? 2. Relevant equations I used the cosine rule here 3. The attempt at a solution As we have 2 sides and an angle I used the cosine rule - Am I correct? (2) means squared b(2) = a(2) + c(2) - 2ac cos B so x(2) =21(2) + (2x)(2) - 2.21.2x.c cos B x(2) =441 + 4x(2) - 84x cos B 3x(2) = 84x cosB - 441 How do I find x from here?