Looking at the equation for the total pay, we can see that the slope of the function is 70.
The slope is the value multiplying the variable x in the function.
For each unitary increase in x, the total value of y will increase by the value of the slope, that is, 70.
Therefore the change in total pay for each copy is $70.
If Rita doesn't sell any copies, the value of x is zero, so the value of y will be:

Therefore the total pay will be $2500.