Answer:
x = 3√7 cm.
or 7.94 cm to the nearest hundredth.
Step-by-step explanation:
We use the Cosine Rule:
21^2 = x^2 + (2x)^2 - 2x.2x cos 120
21^2 = 5x^2 - 4x^2 * -0.5
441 = 5x^2 + 2x^2
7x^2 = 441
x^2 = 63
x = √63
