The answer is F(x) = 900x + 600
Let x stands for the number of months
The apartment is rented for $900 a month, so every month it will be paid 900x (for example, for one month (x = 1), you will pay 900 · 1 = $900, for three months (x = 5), you will pay 900 · 5 = $4500, etc.)
A security deposit is paid only once and is not dependent on the number of months. So, its value is only 600.
These two values must be summed up because both must be paid, nothing is to subtract. Therefore, the function f(x) for apartments that rent for $900 a month and a security deposit of $600 is:
f(x) = 900x + 600
