Question from George, a student:

I have the sequence
50,120,235,406,644

the 1st difference is 70,115,171,238
the 2nd difference is 45,56,67
the 3rd difference is 11,11

i used the a*n^3+b*n^2+c*n+d rule but the a turns out to be a decimal
because 11/6 is 1.833333333 and so i cant find the formula for the 100th term
term. Can someone show me how to find it and tell me the answer because i
really need that for tomorrow.