considering the one time deposit and monthly went rent the equation y equals 1200+400x can be used to represent why the total cost of renting an apartment for x months what is the amount of the deposit

The trick with this problem is that there is no trick - there's no math involved at all, just wordplay. The key is in one-time deposit; what you're looking for isn't a recurring fee, but rather a constant. Now, an equation is made up of three things:

a variable a relational statement in the form of = a constant, even if it isn't really there, it's zero

In this case, what you're looking for is the constant in the equation; a value that doesn't change when any variable changes.

The only number in your question that fits the bill is 1200$, since it's a one-time, unchanging value. y is the total amount paid and x represents the months, which are both variables; 400 is tied to x, so it also changes based on months.