How tall is the tree?

A normal-level "Length" problem.Getting a height you cannot measure from a shadow you can. This is similarity at its most useful.

GivenAt the same moment, a 3 m pole casts a 2 m shadow and the tree casts a 8 m one

Hints

At the same moment the sun's rays are parallel everywhere: they strike the pole and the tree at exactly the same angle.
The pole with its shadow, and the tree with its shadow, both make triangles with a right angle and the same sun angle. All angles match, so they are similar.
Being similar, the ratio "height : shadow" is the same for both: 3 : 2 for the pole, ? : 8 for the tree.
Compare the two shadows: 8 ÷ 2 = 4. The tree's shadow is 4 times the pole's.
In similar shapes every side scales by the same factor. The shadow is 4 times, so the height is 4 times too. 3 × 4

Answer 12m