The diagram shows an isosceles triangle. An isosceles triangle has two sides equal in measurement, and the two corresponding angles are also equal. With this we can identify all three angles (even though only two were identified in the question.
Line AB is congruent to line BC as stated. That makes angle A equal to angle C. If angle A is x, then angle C is also x (rule of isosceles triangle). Remember also that angles in a triangle sum up to 180 degrees, which means that angle A plus angle B plus angle C all equals 180.
Therefore;