Scientific Notation Simplification

$\left(2.7\cdot\:1017\right)+\left(4.2\cdot\:1015\right)\\ \mathrm{Remove\:parentheses}:\quad\left(a\right)=a\\ =2.7\cdot\:1017+4.2\cdot\:1015\\ \mathrm{Multiply\:the\:numbers:}\:2.7\cdot\:1017=2745.9\\ =2745.9+1015\cdot\:4.2\\ \mathrm{Multiply\:the\:numbers:}\:4.2\cdot\:1015=4263\\ =2745.9+4263\\ \mathrm{Add\:the\:numbers:}\:2745.9+4263=7008.9\\ =7008.9$