Counting

Find the positive integer n such that the expansion of (4x - 7y)^n contains a term of the form cx^2*y.

General term is

    $\binom{n}{r}(4x)^r (-7y)^{n-r}\\ r=2,\\ n-2=1\qquad n=3\\~\\ \binom{3}{2}(4x)^2 (-7y)^{1}\\ $

Anyway,    n=3