How do i simplify 5x + -2x + 6x + -x + 3x + 15x. ?

Ineed if you could explain and give me the answer you! this needs done

Eric has challenged himself to walk 24,000 steps in 4 days. if eric walks the same number of steps each day, which function represents the number of steps eric still needs to walk to reach his goal with respect to the number of days since he started his challenge? a. y = 8,000x − 24,000 b. y = -8,000x + 24,000 c. y = 6,000x − 24,000 d. y = -6,000x + 24,000

Find the solution of this system of equation -7x+y=-20 9x-3y=36