Plz help me well mark brainliest if you are correct!

What is the missing reason in step 5? linear pair postulategivendefinition of complementary anglescongruent complements theorem

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.

What is the answer to the question 5x+2x