Please help. Stuck on question

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

Find the domain and range of the following function f(x) = 51x - 21+ 4

Sanderson is having trouble with his assignment. his shown work is as follows: − 3 7 7 3 = − 3 7 × 7 3 = − 21 21 = −1 however, his answer does not match the answer that his teacher gives him. complete the description of sanderson's mistake. find the correct answer.