The image contains a mathematical exercise in Serbian that asks for the calculation of unknown lengths (denoted as $x$) from different geometric configurations. Here's the breakdown of each part:

Problem 39: Calculate the unknown length $x$ from the diagram, given:

a)

$AD = 12 \, \text{cm}, \, DB = 4 \, \text{cm}, \, AE = 18 \, \text{cm}, \, EC = x$

b)

$AD = 8 \, \text{cm}, \, AE = 12 \, \text{cm}, \, AB = 5 \, \text{cm}, \, BC = x$

c)

$AB = 7 \, \text{cm}, \, BC = 3.5 \, \text{cm}, \, AD = 2 \, \text{cm}, \, DE = x$

d)

$AB = 9 \, \text{cm}, \, EC = 4 \, \text{cm}, \, BC = 3 \, \text{cm}, \, DB = x$

Would you like to go through each of these in detail and solve them step by step?

Questions for Further Thought:

1. How do the different segments relate to each other geometrically (e.g., are they part of triangles, trapezoids)?
2. What method can we use to solve for $x$ (e.g., segment addition, Pythagoras theorem, similarity)?
3. Are there any specific properties of triangles or polygons being used in these problems?
4. How can we verify if the calculated length $x$ makes sense in each case?
5. What would happen if any of the known lengths were altered? How would that affect $x$?

Tip:

In geometry problems, always identify whether the figures are triangles, quadrilaterals, or other polygons, as this will help in deciding the correct method for solving.