Helpop thank u

Dylan and dusty plan to take weekly surfing lessons together. if the 2-hour lessons are $20 per person and they plan to spend $100 each on new surfboards, what is the maximum number of lessons the two can take if the total amount spent for lessons and surfboards is at most $480?

Which value is not a solution of 8x^3 – 1 = 0

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