Question 1005420
1. {{{12y^4x^2y^2+8y^3}}} does not look right: if you have a {{{y^4}}} before the {{{x^2}}} , why would there be a {{{y^2}}} after  the {{{x^2}}} ?
Is that what you were expected to "simplify"?
{{{12y^4x^2y^2+8y^3=12x^2y^6+8y^3=4y^3(3x^2y^3+2)}}} 
2. {{{(x^3+27)/(x+3)=(x+3)(x^2-3x+9)/(x+3)=x^2-3x+9}}} for {{{x+3<>0}}}<-->{{{x<>-3}}}
3. Subtract {{{3a^2+4b^3-5c}}} from {{{3a + b^3+5d }}}
{{{3a+b^3+5d-(3a^2+4b^3-5c)=3a+b^3+5d-3a^2-4b^3+5c=3a+5d-3a^2-3b^3+5c}}}
4. {{{24x^3y-30xy^2+48x^3y^3z=6xy(4x^2-5y+8x^2y^2z)}}}
5. {{{3a-2b+(3a+2b)(3a-2b)=(1+3a+2b)(3a-2b)}}}