In this specific question, the first thing you should do is to figure out what k is. You can do this by using the fact that the half life of carbon-14 is 5700 years. To find k set up your equation as follows:
.5=e^5700k
To figure out k here take the natural log of both sides:
ln(.5)=5700k
Then solve for k:
k=ln(.5)/5700 ~~ -1.216*10^-4
Now that we know what k is, we can go back to the original question and plug in everything we know! Our new equation should look like this:
.04=e^((ln(.5)/5700)t)
Solving for t works the same as solving for k, take the natural log of both sides:
ln(.04)=(ln(.5)/5700)t
Then solve for t:
t=ln(.04)/(ln(.5)/5700) ~~ 26469.98 years