The pattern of coefficients is like pascals triangle, the outside is always 1's and you add the two coefficeints in the line directly above to get the inside ones:

...1 1

..1 2 1

.1 3 3 1

1 4 6 4 1

As for the 3 (or y value see above) when you are talking about a power of 7 the power of the y will always be 7 - power of x in that part of the polynomial.

Try to figure it out and post the solution if you can't get it or if you want me to check it I will give the answer.