Scott started his banking account with $150 and is spending $7 per day on lunch. the x axis would be labeled:

For this case, the first thing you should do is define variables:
 x: number of days
 y: money available
 We now write the linear function that represents the problem:
 y = -7x + 150
 Therefore, the x axis is labeled as:
 Number of days.
 
 The x axis would be labeled:
 Number of days