Bob gets paid an annual salary of $30,000 and earns 5% commission on all sales he makes. if bob wants to make $6,000 this month, how much in sales does he need to have?

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