The length of the third side of the triangle is approximately 5.2 cm.
To measure the length of the third side of the triangle, we can use the Law of Sines. The Law of Sines states that the ratio of the sides of a triangle is equal to the ratio of the sines of the opposite angles.
In this case, we know the length of one side (7 cm) and the measure of two angles (40° and 20°). We can use the following equation to solve for the length of the third side (x):

Solving for x, we get:
x =

Therefore, the length of the third side of the triangle is approximately 5.2 cm.