Evaluate 2a+2b when a=10.8 and b=8.3

Josh has a rewards card for a movie theater he receives 15 points for becoming a rewards card holder he earns 3.5 points for each visit to the movie theater he needs at least 55 points to earn a free movie ticket which inequality can josh use to determine x, the minimum number of visits he needs to earn his first free movie ticket.

What is the value of x in the following equation. -x ^3/2 = -27

We just started the introduction into circles and i have no idea how to do this.