I’m rlly dumb and i don’t get these at all can someone explain these

Which is an equation for the nth terms of the sequence 12,15,18,21

Add the fractions, and simply the result to lowest terms. 2/3 + 2/7= ?

Which of the expressions (on the photo document i inserted) together represent all solutions to the following equation? 8cos(12x)+4=-4 *the answer should be in degrees.* > i would like for someone to explain the process to find the solutions for this.