that's because Cosine is adjacent to the angle (the angle being 30 degrees and the side next to it is 43mm) divided by the hypotenuse (longest side of a right triangle. The 50 mm). A way you can remember the sin, cosine and tangent is SOH CAH TOA. Sine=opposite over hypotenuse, Cosine = adjacent over hypotenuse, Tangent = opposite over adjacent. For the next one, it is the same principal. X is adjacent to 22 degrees and the other number is the hypotenuse so it has to be Cosine.
