Hi, i need a little help with a question that i am getting stuck on.

It has to do with the binomial theorem.

If i could get some help to put me on the right track that would be a great help.

Thanks

That's because you have the wrong R. R cannot be negative. Notice that each of the variables is to the 4th power inside the parenthesis. Thus
$\displaystyle R = 50 - \frac{100}{4} = 25$