To solve for $EF$, we can analyze the given segments and apply geometric reasoning.

Given:

• $BC = 4$
• $DE = 5$
• $FB = 3$

Assuming that these segments are part of a figure like a triangle or other geometric shapes, more specific information about their arrangement is needed. If these are parts of a triangle, we could use rules like the Pythagorean theorem or the properties of similar triangles.

Since it's not clear how these segments are arranged, could you clarify whether this is part of a triangle, trapezoid, or another geometric figure?

Would you like to clarify the setup or explore potential solutions for different cases?

Further related questions:

1. Are $BC$, $DE$, and $FB$ parts of a triangle?
2. Are these line segments connected, or are they from different triangles?
3. Should we apply the Pythagorean theorem for this problem?
4. Is $EF$ a diagonal or a side of a polygon?
5. Are there any angles provided that could help in solving this?

Tip: When solving for unknown sides, knowing the type of geometric figure you're working with can guide the application of the right theorem (like Pythagoras or similar triangles).