These fossils are important, and scientists are interested in them because plants can be identified by their flowers.
Adverb clause best identifies the clause "because plants can be identified by their flowers. "

Many ancient Egyptian tombs, which were discovered centuries before, had been robbed of their treasures.
The clause "which were discovered centuries before" modifies TOMBS.

Now the promoters of the Vikings-first theory are getting a taste of their own medicine. This is a simple sentence.

u^-3=1/125
1/u^3 = 1/125
1/u^3 = 1/5^3
u = 5

u - 3 = 1/125;
u = 1/125 + 3;
u = 376/125;
u = 3 1/125

(2.6 × 10^–4)(2.2 × 10^8)
= 2.6*2.2*10^-4*10^8
= 5.72*10^4