Find the common difference of the arithmetic sequence -7, -1, 5, ...

I've never been good at math he told he loves but he keeps cheating on me, so it ain't adding up sis

Suppose that the price p (in dollars) and the weekly sales x (in thousands of units) of a certain commodity satisfy the demand equation 4p cubedplusx squaredequals38 comma 400. determine the rate at which sales are changing at a time when xequals80, pequals20, and the price is falling at the rate of $.20 per week.

If f(x) =3x-2 and g(x) =2x+1 fins (f+ g) (x)