The x seems to represent the number of months that Leslie has joined the gym for. When x = 0 (basically when she hasn't started using her membership), she pays 55(0) + 50 = $50 so we know that the 50 represents the one-time fee. The only thing missing is the monthly fee and since we haven't defined what the 55 means, we can conclude that the 55 represents the monthly fee. This makes sense because 55x is simply 55 times x, and since x is the number of months, 55 times that would be how much Leslie pays per month, otherwise known as the monthly fee.
