You should use the cosine theorem. Let's construct a triangle in a fashion like in the fig.below.
Triangle.png
Here, the angle B = 55 grades, AB = 58 cm, BC = 70 cm, and AC the unknown side. Using the cosine theorem where the angle B is located in-between AB and AC we get
BC^2 = 58^2 + 70^2 - 2*58*70*cos (55 grades) = ...
+259 
