Think about the expansion itself. As it is to the power of 3 this means that when you continue to use the binomial formula terms after x^3 will always be equal to zero (try it out). This is because if the power you are exploring is an integer then the expansion will work for all values of x. It is the same as rewriting (1+2x)^3 as (1+2x)(1+2x)(1+2x) which works for all x. If it still isn't clear tell me what you don't understand and i'll try to explain further.