feat : solution for symbl.ai GitHub-externship Assignment

[kumarSuraj-bit]

Algorithm :

        step 1: finding coefficients A, B, C, and D from the coming equation.

        step 2: then find coefficient In which '?' is present, assume that a variable 
                and then calculate that variable by simple math.

        step 3: now compare that calculated variable to the given equation coefficient which 
                contain '?' by our defined method isPossible() it will give result whether 
                our solution is possible or not.

        step 4: If the solution is possible for a given equation, we return that digit which is 
                missing, otherwise, we return -1.