Tangent is opp/adj. In this case, we are given the adj and need to find the opp. Let's set up a ratio:
[tex]tan(25) = \frac{AC}{8.6} [/tex]
Solve for AC:
[tex]8.6tan(25) = AC = 4.01[/tex]
So, since it says round to the nearest tenth, your answer is AC = 4.0 or 4 units.