Step 1: Remember the law of sine and law of cosines formulas and properties.

Link: http://mathematics.laerd.com/maths/trigonometry-sine-and-cosine-rules-intro.php

Step 2: Look at the given measurements and identify the suitable law for the question.

NOTE: If you know the measures of two angles and the length of one's opposite side, you can use the Law of Sines to solve for the length of the other angle's opposite side.

EXAMPLE: Since you know the measure of \angle X, the length *x* of its opposite side, and the measure of \angle Y, you can use

the **Law of Sines** to find the length *y* of the side opposite \angle Y.

\frac{y}{\sin Y} = \frac{x}{\sin X}

Since you only know one side length, you can't start with the Law of Cosines.