$a)\left(6\cdot {10}^7\right)\cdot \left(2,5\cdot {10}^{-3}\right)=6\cdot 2,5\cdot {10}^{7+\left(-3\right)}=15\cdot {10}^4=1,5\cdot 10\cdot {10}^4=1,5\cdot {10}^5$

$b)\left(4,5\cdot {10}^{-5}\right)\cdot \left(4,4\cdot {10}^{-11}\right)=4,5\cdot 4,4\cdot {10}^{-5+\left(-11\right)}=19,8\cdot {10}^{-16}=1,98\cdot 10\cdot {10}^{-16}=1,98\cdot {10}^{-15}$

$c)\left(8,4\cdot {10}^3\right):\left(2,1\cdot {10}^{-7}\right)=\frac{8,4\cdot {10}^3}{2,1\cdot {10}^{-7}}=\frac{8,4}{2,1}\cdot \frac{{10}^3}{{10}^{-7}}=\frac{84}{21}\cdot {10}^{3-\left(-7\right)}=$   $=4\cdot {10}^{3+7}=4\cdot {10}^{10}$