you can do this by identities, or using triangles. lets use identities here.
you should know that
you know the value of , so plug it in and solve for . you will get two answers, a negative one and a positive one, but they want the positive, since they said
to do it by triangles, you can remember that sine = opposite/hypotenuse. so you can draw a right-triangle, with an acute angle with the opposite side having a length of 2 and the hypotenuse having a length of 3. then you can use Pythagoras' theorem to find the length of the adjacent side. now cosine = adjacent/hypotenuse. just plug in the values you found
By the way, it is "theta" not "vada"