Use cosine ,, think of it this way SOH CAH TOA, SOH meaning Sin= opposite/hypotenuse, CAH meaning Cos= adjacent/hypotenuse and TOA meaning tan= opposite/ adjacent so in your diagram 6 is the adjacent because the angle on that line are 90 and 20 degrees so basically if you see a 90 degree and another angle on that line it is an adjacent and hypotenuse line is always opposite the 90 degree hence we will use the function cosine
