The firm will sell 1,264 family memberships and 1,920 corporate memberships.
The firm will charge -0.5 for family memberships and 0.2 for corporate memberships.
The total profit that the firm will make is -90.
How to solve
1. Determining the Number of Memberships
To maximize profit, the firm should set the prices for each type of membership such that the marginal revenue from each type is equal to the marginal cost. The marginal revenue for each type of membership can be found by taking the derivative of the demand function with respect to price:

The marginal cost is simply the cost of producing one additional unit of output, which is 10 in this case. Therefore, we have the following system of equations:

Solving for the prices, we get:

Now, we can plug these prices back into the demand functions to find the quantities:

Therefore, the firm will sell 1,264 family memberships and 1,920 corporate memberships.
2. Calculating the Prices
The prices for each type of membership have already been calculated in step 1:

3. Determining the Profit
The profit for each type of membership can be found by subtracting the marginal cost from the marginal revenue:

The total profit is the sum of the profits from each type of membership:

Therefore, the total profit that the firm will make under third-degree price discrimination is -90.