As the equations show, national saving (S) is composed of private saving (sp) and public saving (Sc). The economy's output (GDP) is depicted as Y. Government revenue is realized from tax (T), which is a sum of indirect tax (to) and direct tax (n). Furthermore, government spending is purely exogenous and represented by Go. Planned investment is in three parts: the startup investment (lo), part of investment that depends on income (I), and part that depends on interest rate (r). Equilibrium in the real (goods) market requires that planned investment (I) is equal to national saving (S) (equation 1). The balance of payments (BP) will be at equilibrium when the interest rate is in accordance with the BP relation you have derived.