Which statement shows the associative property of addition?

a.
if m + 3 = 18, then 18 = m + 3

b.
(u + 7) + 13 = u + (7 + 13)

c.
21 + 2y = 2y + 21

d.
3 · (5 · x) = (3 · 5) · x

Okay so i didn't get this problem petro bought 8 tickets to a basketball game he paid a total of $200 write an equation to determine whether each ticket cost $26 or $28 so i didn't get this question so yeahyou have a good day.