[tex]\rule{6cm}{1mm}[/tex]
[tex]A=3(12+3a)[/tex]
[tex]A=3\times 12 + 3\times 2a[/tex]
[tex]\boxed{\blue{A=36+6a}}[/tex]
[tex]\rule{6cm}{1mm}[/tex]
[tex]B=5(8x-6)[/tex]
[tex]B=5\times 8x + 5\times (-6)[/tex]
[tex]\boxed{\blue{B=40x-30}}[/tex]
[tex]\rule{6cm}{1mm}[/tex]
[tex]C=4y(9-3y)[/tex]
[tex]C=4y\times 9 + 4y\times (-3y)[/tex]
[tex]\boxed{\blue{C=36y-12y^{2}}}[/tex]
[tex]\rule{6cm}{1mm}[/tex]