Help i don’t understand!!

Which expression is equivalent to (3^2)^-2 a.-81 b.-12 c.1/81 d.1/12

Sanya noticed that the temperature was falling at a steady rate of 1.4 degrees every hour from the time that she first checked her outdoor thermometer. by 6 a.m., the temperature had fallen 21 degrees. which expression can you use to find how many hours earlier she had first checked the thermometer?

Can someone me with #s 8, 9, and 11. with just one of the three also works. prove using only trig identities.