Since the degree given to you is 25 you can identify the three sides. The longest side opposite to the 90 degree angle is the "hypotenuse", the shortest side is the "opposite", and the side that is 120 m is the "adjacent".
Once you know the names of the three sides you just have to remember SoaCahToa. Meaning Sin is opposite over adjacent, Cosin is adjacent over hypotenuse, and Tangent is opposite over adjacent. The two sides you were given are x (the hypotenuse) and 120 (the adjacent) so you use Cosin.