$a)\left(2\cdot {10}^{15}\right)\cdot \left(3\cdot {10}^{-3}\right)=2\cdot 3\cdot {10}^{15+\left(-3\right)}=6\cdot {10}^{12}$

$b)\left(5,5\cdot {10}^{-6}\right)\cdot \left(8\cdot {10}^{14}\right)=5,5\cdot 8\cdot {10}^{-6}\cdot {10}^{14}=44\cdot {10}^{-6+14}=44\cdot {10}^8=4,4\cdot {10}^9$

$c)\left(6,3\cdot {10}^{14}\right):\left(7\cdot {10}^{13}\right)=\frac{6,3}{7}\cdot {10}^{14-13}=\frac{6,3}{7}\cdot 10=\frac{63}{7}=9=9\cdot 1=9\cdot {10}^0$

$d)\left(9,6\cdot {10}^{-8}\right):\left(1,6\cdot {10}^{-12}\right)=\frac{9,6}{1,6}\cdot {10}^{-8-\left(-12\right)}=6\cdot {10}^4$