$a)7000000=7\cdot {10}^6$

$230000=2,3\cdot 100000=2,3\cdot {10}^5$ $230000=2,3\cdot 100000=2,3\cdot {10}^5$

$65080000=6,508\cdot 10000000=6,508\cdot {10}^7$

$b)0,2=0,2\cdot 1=0,2\cdot {10}^0$

$0,0075=7,5\cdot 0,001=7,5\cdot {10}^{-3}$

$0,00000504=5,04\cdot 0,000001=5,04\cdot {10}^{-6}$