Identify the angle measurements in both triangles. Place a protractor on each angle, center the hole on the base of the protractor over the vertex and line up the base with one side of the angle. The angle's other line will touch one of the numbers along the protractor to indicate the size of the angle in degrees.
Establish a proportion between the two triangles to find the missing length. For example, the first triangle has side lengths of 12, 9 and 2, and the second triangle has side lengths of 24, x, and 4. These side lengths are arranged by angle for both triangles, so that the first triangle's 12 unit side corresponds to the second triangle's 24 unit side. The proportion would look like this: 12/24 = 9/x.
Cross-multiply to solve for the missing length. In the example, 12x = 216. Divide both sides by 12 to get 18, which is the missing length.