To determine the measure of side $b$, we need to analyze the triangle. The given triangle has:

• Hypotenuse $c = 10$.
• Angle at $A$ is labeled, suggesting we might use trigonometric relationships.

If the triangle is a 30°-60°-90° triangle, standard ratios of the sides apply:

1. The side opposite 30° is half the hypotenuse ($b = \frac{c}{2}$).
2. The side opposite 60° is $\frac{\sqrt{3}}{2}c$.

Alternatively, for general triangles, trigonometric functions like sine or cosine can help:

• $\cos \theta = \frac{\text{adjacent}}{\text{hypotenuse}}$ or
• $\sin \theta = \frac{\text{opposite}}{\text{hypotenuse}}$.

Let me know if further clarification or additional information is needed for calculations!