one of the terms in the expansion is . determin the value of k.
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.