TMUA 2018 Paper 1 Question 19

A triangle $ABC$ is to be drawn with $AB=10$ cm, $BC=7$ cm and the angle at $A$ equal to $\theta$, where $\theta$ is a certain specified angle. Of the two possible triangles that could be drawn, the larger triangle has three times the area of the smaller one. What is the value of $\cos \theta$?