feat: Solution for Github-Externship Assignment

[dev-amrit460]

Algorithm:

           Step1: Finding A,B,C,D from the equation given.

           Step2: using a function called findCoeff() to find in which coefficient, ? is present.

           Step3: Considering the coefficient which contain ? as a veriable, we found the actual value 
                       for that veriable using simple math.

           Step4: If length of calculated value and initially provided value is not same then returning -1 
                       else returning the digit of calculated value at index of ? in initially given value.