Find the positive integer n such that the expansion of (4x - 7y)^n contains a term of the form cx^2*y.
The answer is 3.
This is because if one term is in the form of an x^2, the lowest power needs to be 2. However, since we also have a y, we know the equation can't be a quadratic (think it through!), and therefore the only power sufficing this form would be 3.
Does that make sense?