Open the square brackets: 6xy-6x^2. Add on 12xy=18xy-6x^2, then add on x^2=18xy-5x^2. That gives us the contents of the curly brackets. x is a common factor here, so we could write x(18y-5x). We now have to multiply by what's outside the curly brackets. We can start by combining the 3 with x to produce 3x then the expression in round brackets. Let's focus on multiplying the contents of the round brackets by the contents of the curly brackets: we can write this as 4xy(18y-5x)-4x^2(18y-5x)-3(18y-5x)=72xy^2-20x^2y-72x^2y+20x^3-54y+15x=20x^3-92x^2y+72xy^2+15x-54y. We still have to multiply by -3x before we can add on -6xy. So we have -6xy-60x^4-276x^3y+216x^2y^2+45x^2-162xy=-60x^4-276x^3y+216x^2y^2+45x^2-168xy. Phew! We can't take this any further because we have two variables or unknowns, x and y, and no equation(s).