A salesman gets paid on commission, and his commission is based on how many units he sells during the month. If the salesman sells $1500 worth of units or less, he makes a flat commission of $900 plus 10% of the sale. If he sells more than $1500 worth of units, he makes an additional 6% on anything above $1500. If Mark's months earnings E were $2100 for this month, what was the total value of his sales, s?

To find the total value of Mark's sales, we need to analyze the information provided and create an equation.

Let's break down the information given:

- If Mark sells $1500 worth of units or less, he receives a flat commission of $900 plus 10% of the sales.
- If Mark sells more than $1500 worth of units, he receives an additional 6% on anything above $1500.
- Mark's earnings for this month were $2100, represented as E.
- We need to find the total value of Mark's sales, represented as s.

First, let's calculate Mark's commission for sales of $1500 or less.
The commission for sales of $1500 or less is a flat commission of $900 plus 10% of the sales.
So the commission for sales less than or equal to $1500 can be calculated using the equation:
Commission_1 = $900 + 0.10s

Now, let's calculate Mark's additional commission for sales above $1500.
Any sales above $1500 earn Mark an additional 6% commission.
This can be calculated using the equation:
Commission_2 = 0.06(s - $1500)

Next, let's calculate Mark's total commission for the month.
If Mark's earnings for the month were $2100, then his commission is equal to his earnings.
So we have the equation:
Commission_1 + Commission_2 = E = $2100

Combining all the equations, we have:
$900 + 0.10s + 0.06(s - $1500) = $2100

Now, let's solve this equation to find the value of s, the total value of Mark's sales.