I tried and ended up with t=20 degrees. However I have a feeling that isn't right.
2Cos(3t)-1=0
Cos(3t)=1/2
Cos(3t)=Cos(pi/3)
[if cos(a)=cos(b) then
a=2n(pi)+-b where n is any integer]
3t=2n(pi)+-(pi/3)
t=(2/3)n(pi)+-(pi/9)
here n is any integer (like 0,1,-1 etc.)
This example here is very similar to your question (except it's sine), but im sure you can figure it out from the example. SOLVING TRIGONOMETRIC EQUATIONS