Hey I need help with a problem regarding a septangular.

In a regular septangular there are two diagonals with 2 different lengths. How long is the longer one if the shorter one is 5.0 cm?

The answer is 6.2 cm. To think about is that it can be divided into 5 triangles and the sum of the angles is 900^{o I think I have to use the law of sines and cosines. Im really stuck, I appreciate the help }