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## Homework Statement

In triabgle ABC, AB = 2x cm, AC = x cm, BC = 21 cm and angle BAC = 120 degrees. Calculate the value of x?

## Homework Equations

I used the cosine rule here

## 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?