ANSWER
8x + 3y = 2000
x => The number of adults
y => The number of children
EXPLANATION
Let the number of adults be x.
Let the number of children be y.
Each adult will be charged $8 admission. This would mean that the charges for x adults will be:
8 * x = $8x
Each child will be charged $3 admission. This would mean that the charges for y children will be:
3 * y = $3y
The club wants to earn $2000 from admission charges. This means that adding the total charges for adults and children should yield $2000.
Therefore, we have:
8x + 3y = 2000
The variables are:
x => The number of adults
y => The number of children