Explain how you would solve a triangle given two sides and an included angle.

Use the Law of Cosines, since you have 2 side lengths and the measure of the included angle.

The third side could be named "a."  Then, according to the Law of Cosines,

a^2 = b^2 + c^2 - 2(a)(b) cos C, where C is the included angle, and a=b in this particular problem.  Thus,

a^2 = 2b^2 - 2ab cos C.