In triangle ABC it is known that AB = BC, angle ABC = 124 degrees. Find the BCA side.

If two sides in a triangle are equal to each other (AB = BC), then such a triangle (ACB) is isosceles. In an isosceles triangle at the base, that is, the third side, the angles are the same.
Since the sum of all the angles of any triangle is 180 degrees, we find what the angle of the BCA is equal to:
∠ BCA = (180 ° – ∠ ABC): 2 = (180 ° – 124 °): 2 = 56 °: 2 = 28 °.
Answer: ∠ BCA = 28 °.