The key characteristic of a linear equation is having a constant slope all throughout. So, when you plot the worker's average pay along the x-axis, and the company's profits along the y-axis, then the slope is equal to Δy/Δx. Since the change in y is a 'drop', then its sign would be negative. Since the change in x is an 'increase', then the sign would be positive.
Slope = (-$1500/month * 12 months/1 yr)÷($450/yr) = -40
