A formula goes from inputs to a result. Goal Seek goes the other way: you name the result you want, and Excel changes one input until the formula gets there.
Data > What-If Analysis > Goal Seek (Alt, A, W, G) asks for three things:
- Set cell: the formula cell whose result you want, such as the monthly payment.
- To value: the number you want it to show.
- By changing cell: one input cell (a typed value, not a formula) that the result depends on.
Excel tries values until the result matches, shows what it found, and keeps the answer when you click OK (Cancel puts the input back). It changes the input to a plain number, so check it makes sense and round it if the real-world value must be whole.