Step-by-step explanation:
Well. Let's call α the 120° angle, and β its supplementary angle (80°). Let's draw also the height of the triangle. We'll call it CH. We know, from goniometric relations, that BC * sinβ = CH and BC * cosβ = BH. We just apply the pythagorean theorem so we get: AC = [tex]\sqrt{(AB+BH)^2+CH^2}.[/tex] By inserting all the values you'll get the result.
