What's the name for the difference between ask price and bid price? Use letter keys to select choices
A

Strike price.
B

Margin.
C

Last price.
D

Spread.

Really need on these. i have no idea what to do

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.

Find the value of x for which the lines are parallel.