When you solve a problem involving money, what can a negative number represent

Evaluate the expression for the given value of the variable 7a - 4a for a =8

The following frequency table relates the weekly sales of bicycles at a given store over a 42-week period.value01234567frequency367108521(a) in how many weeks were at least 2 bikes sold? (b) in how many weeks were at least 5 bikes sold? (c) in how many weeks were an even number of bikes sold?

How do you solve a question like this?