The answer is 70,000 sales.
Bob gets paid an annual salary of $30,000, which means that monthly he gets:
$30,000 : 12 months = $2,500 per month
x - number of sales
Bob earns 5% commission on all sales he makes: x * 5% = x * 0.05 = 0.05x
The function is:
f(x) = 2,500 + 0.05x
Bob wants to make $6,000 this month: f(x) = 6,000
2500 + 0.05x = 6000
0.05x = 6000 - 2500
0.05x = 3500
x = 3500 : 0.05
x = 70000 sales