Final answer:
The equation for autonomous expenditure is 800. Autonomous expenditure is the initial level of expenditure that does not depend on income or output.
Step-by-step explanation:
The question is asking for the equation for autonomous expenditure. In the given equation, PAE = 800 + 0.9Y, the autonomous expenditure is equal to 800. Autonomous expenditure refers to the part of total expenditure that does not depend on income or output. In this case, it is the initial level of expenditure, which is 800.