Final answer:
Without additional context provided in the question, θ cannot be determined just with the given values of a and b. Trigonometry or vector analysis would be needed, often involving the cosine rule or vector dot product.
Step-by-step explanation:
Given a = 3 cm and b = 21 cm, to work out the angle θ between them if these represent vectors or sides of a triangle, we would typically use trigonometry or vector operations. However, based on the provided information, there isn't a direct way to determine the angle θ without additional context or formulas such as those related to vector dot products or the cosine rule for triangles. More information would be needed to provide a step-by-step solution. In tasks involving vector addition and subtraction, protractors and rulers can be used to construct parallelograms and measure angles and lengths to find resultant vectors, as seen in the provided examples.