Using row 4:
coefficients are: 1, 4, 6, 4, 1
a^4 + a^3b + a^2b^2 + ab^3 + b^4
Now adding the coefficients:
1a^4 + 4a^3b + 6a^2b^2 + 4ab^3 + 1b^4
Substitute a and b:
a = 4x
b = -3y
1(4x)^4 + 4(4x)^3(-3y) + 6(4x)^2(-3y)^2 + 4(4x)(-3y)^3 + 1(-3y)^4
Now simplify the above:
256x^4 - 768x^3y + 864x^2y^2 - 432xy^3 + 81y^4