A 2.5 m pole casts a shadow that measures 3m. A nearby tree casts a shadow that is 8.4 m long. Find the height of the tree.
The best way to do this problem is to construct similar triangles.
When you construct the triangles, the ratios of the heights and lengths should be proportional.
So if we let $\displaystyle h$ represent the height of the tree, we see that $\displaystyle \frac{2.5}{h}=\frac{3}{8.4}$ (or we could have done $\displaystyle \frac{2.5}{3}=\frac{h}{8.4}$; try to see why)
Can you try to finish from here?