Hello! I don't really understand what the following question means.

Suppose the following model represents supply and demand in the market for laptop computers:

Q_{d =}a - bP + cY ? dP_DT

Qs =eP - fPM

let a = 300, b = 10, c = 40, d = 30, e = 300, and f = 100.

Q: Which sign (= or -) should replace the question mark in the demand equation? Why? What would a plus or minus sign signify in the laptop computer market?