Answer:
12 + x ≤ 21
Up to 9 more people can ride the bus
Explanation:
Since we're told that the bus only has 21 seats, 21 people at most can ride the bus. Taking into account the 12 people already on the bus, we want to find a value for x (additional riders) that either makes the total amount of the people on the bus equal to or less than 21.
Thus, the equation we need to use is 12 + x ≤ 21.
(12 + x ≤ 21) - 12
x ≤ 9
Thus, up to 9 more people can ride the bus.