The side y is opposite the angle of 57 degrees. The side 15 is the hypotenuse. Thus we use the sine ratio which is opposite/hypotenuse.

Use the T=O/A, C=A/H, S=O/H method. Memorise it as "toa cah soh"(meaning Big Foot Lady in hokkien):)

Firstly, label all the sides of the right angled triangle. Assign Opposite, Adajacent and Hypothenus to each side. Look at what you are given and what must be found. In this case, you must find 'y', which is the "O(opposite)" and you are given the Angle 57 degrees and H=15. Therefore, use "S=O/H".

Hope this method helps you:)

Using sin angle equals to opposite over hypothesis