This is a standard VLOOKUP formula. It requires a table with lookup values (in this case, dates) to the left of the values being retrieved.
The lookup value comes from cell E6, which must be a valid date. The table array is the range B6:C11, and the column index is 2, since the amounts are in the second column of the table. Finally, zero is provided for the final argument to force an exact match.
The VLOOKUP function locates the date value for Sept 4, and returns the value at the same row in the second column: 12,500.
Note: the lookup value and the date values in the table must be valid Excel dates.