How Do I Calculate the Time Constant from Oscillation Decay?

[LBRRIT2390]

Homework Statement

A gust of wind hits a flagpole and starts it oscillating. After 10 s, the amplitude of its motion has decreased to 25% of its initial value.

What is the value of the time constant?

Homework Equations

x_max(t) = Ae^$$-t/\tau$$

The Attempt at a Solution

I know that the answer is 7.2s, but how do I find it?

How do I manipulate the above equation?

 

[willem2]

Use the equation you gave at with t=0 and also with t=10