When I took this course, we used a funny little expression to remember how to calculate Sin, Cos and Tan: SOH -CAH-TOA. In this problem, we use Cos from the angle with 20 degrees (if you do it with the 90 degree angle, you get 0 because the triangle does not exist.)
Brainly is hard to format this kind of stuff, so I'll just show you my handwritten work. Value y is the hypotenuse of the triangle, 10 cm is the adjacent. Using the expression Cos20 = 10/y and you get y =~ 10.6 cm.
