Answer:
y = 8
Explanation:
The various segments of the diagram are proportional from one triangle to the other. Here, we see that the length 10 is twice the difference between the long sides:
(10 = 2·(15-10))
Then y will be twice the difference between the lengths of the bottom edges. That difference is marked as 4, so y = 2·4 = 8.
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Side note
The relative dimensions of the sides of the triangles are 3:4:5, so this will actually be a right triangle, with the right angle at lower left. It is not drawn to scale.
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If you insist on equations, you can write ...
y/(y+4) = 10/15 . . . . corresponding sides are proportional
15y = 10y +40 . . . . multiply by 15(y+4)
5y = 40 . . . . . . . . . subtract 10y
y = 8 . . . . . . . . . . . divide by 5