If actual real GDP is greater than the equilibrium level of real GDP (i.e., the aggregate expenditure function is below the 45-degree line), how is equilibrium restored?

At the current level of real GDP, planned spending (C + I) is less than production. The amount of goods
and services produced is greater than the amount people intend to buy. Inventories rise above the levels
firms desire to hold. These unintended inventory increases are a signal to firms to reduce production.
Production will continue to fall until desired inventory levels are restored or when the amount firms
produce just equals aggregate expenditure.