how is saving and investment approach …

Equilibrium means the state of balance or state of no change. The equilibrium level of income or output is that level at which the planned savings and planned investments are equal.
There are two alternative approaches of national income determination and the first approach determines equilibrium level by the equality of aggregate demand and aggregate supply of output.
It is derived from the Aggregate Demand and Supply approach in the following way:
In a simple economy, there are two elements of national income consumption and investment. An economy is said to be in equilibrium when aggregate expenditure equals aggregate income or aggregate money value of all goods and services. Aggregate Demand (AD) in a two-sector economy is defined as the sum of Consumption Expenditure and Investment Expenditure (I) i.e. AD = C + I, whereas Aggregate Supply (AS) is defined as the sum of Consumption (C) and
Saving (S) i.e AS = C + S.
Mathematically, AD = AS
or, C + 1 = C + S
Hence, I = S
or S = I

In the graph given, OP or OP' is the equilibrium level of income E is the equilibrium point where Aggregate Demand equals Aggregate Supply Equality between AS and AD implies the equality between S and I. When we extend the line EP vertically downward, it meets at point E' with S and L It is the equilibrium point of saving and investment approach OP or OP' represents the level of income at which the economy is in equilibrium.