Let's say you you know the desired result of some formula, but you need to find possible input value(s) to achieve that result. You may even know the approximate values of those parameters. However, it's probably nothing more than guessing, and you need exact values.
In such cases it's better to use Excel's Goal Seek tool which helps to arrive at exact solutions very efficiently.
I'm providing here an example of using the Goal Seek. The example shows how to setup your worksheet for solving a cubic equation, such as Ac*x^3+Bc*x^2+Cc*x+Dc=0, where I renamed the coefficients a,b,c and d to Ac,Bc,Cc and Dc.
In this case we want to find the roots for this specific equation: