The average of anything is calculated by adding up all the values and dividing by the number of values.
The Average = 12,000
the # of values = 4
x = score of the 4th game
(11,000 + 11,000 + 11,000 + x) / 4 = 12,000
First, let's multiply both sides by 4
11,000 + 11,000 + 11,000 + x = 48,000
Let's simplify the left side
33,000 + x = 48,000
Subtract 33,000 from both sides
x = 15,000
You scored 15,000 points in the 4th game