Using the order of operations, what should be done first to evaluate?

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