Expand (4x-3y)^4 using pascal's triangle

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 

So confused right here i need it to be more clearer