(x-p)(x-q)
y(x-q) .... let y = x-p
yx - yq ... distribute
x(y) - q(y)
x(x-p) - q(x-p) ... replace y with x-p
x^2 - px - qx + pq .... distribute
x^2 + (-p-q)x + pq
The last expression is in the form ax^2+bx+c with a = 1, b = -p-q and c = pq
The constant term is the term without any variable x attached to it (either x or x^2), so the constant term is pq
