$a)$  

$5000=5\cdot {10}^3$  

$12000000=1,2\cdot {10}^7$  

$34000=3,4\cdot {10}^4$  

$12800000=1,28\cdot {10}^7$  

$231000000000=2,31\cdot {10}^{11}$  

$14040=1,404\cdot {10}^4$  

 

 

$b)$  

$0,000001=1\cdot {10}^{-6}$  

$0,0000009=9\cdot {10}^{-7}$  

$0,00000000012=1,2\cdot {10}^{-10}$   

$0,000000000000045=4,5\cdot {10}^{-14}$  

$0,00104=1,04\cdot {10}^{-3}$  

$0,000002343=2,343\cdot {10}^{-6}$