Find the positive integer n such that the expansion of (4x^2 - 7y)^n contains a term of the form cx^2*y^6.
n=7
Expand (4x^2 - 7y)^7
16384 x^14 - 200704 x^12 y + 1053696 x^10 y^2 - 3073280 x^8 y^3 + 5378240 x^6 y^4 - 5647152 x^4 y^5 + 3294172 x^2 y^6 - 823543 y^7
