Where is the problem? Do you know the derivative of ln(u) where u is a function? ...
You have to use the 'chain rule'.
You can follow what Archie meade said or:
Let u=t^8(t^7-2)=t^15-2t^8
So:
D(ln(u))=D(u)/u=(D(t^15-2t^8))(/t^15-2t^8)
If you can calculate this derivative: D(t^15-2t^8) then you are finished (you can also simplify the fraction).