\[\boxed{\text{534\ (534).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ b^{15}\ :b^{12} = b^{15 - 12} = b^{3}\]
\[\textbf{б)}\ 7^{39}\ :7^{13} = 7^{39 - 13} = 7^{26}\]
\[\textbf{в)}\ a^{11}\ \ :a = a^{11 - 1} = a^{10}\]
\[\textbf{г)}\ 12^{100}\ :12^{99} = 12^{100 - 99} = 12^{1}\ \]