Final answer:
Without the length of side AC or the ratio of the sides in similar triangles ABC and A'B'C', we cannot determine the exact length of AB by proportions.
Step-by-step explanation:
To determine the length of side AB in triangle ABC, which is similar to triangle A'B'C', we can use the proportions property of similar triangles. As we do not have the measure of side AC, which corresponds to side A'C' in triangle A'B'C', we cannot directly relate BC to B'C' and AC to A'C'. Therefore, we cannot provide the exact length of AB without further information about side AC or the ratio of the sides of the triangles ABC and A'B'C'. If such a ratio is provided, we could then apply the properties of similar triangles to find the length of AB.