if you think no one can answer it, why did you ask?

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"