(x2y+3)7.
First we need the coefficients from Pascal's Triangle: 1, 7, 21, 35, 35, 21, 7, 1.
Next we expand (a+b)7, using these coefficients:
a7+7a6b+21a5b2+35a4b3+35a3b4+21a2b5+7ab6+b7.
Let a=x2y and b=3:
x14y7+7x12y6(3)+21x10y5(32)+35x8y4(33)+35x6y3(34)+21x4y2(35)+7x2y(36)+37,
x14y7+21x12y6+189x10y5+35x8y4(33)+945x6y3+5103x4y2+5103x2y+2187.