Expanded notation means you look at each digit individually. So 15 is 5 in the ones place and 1 in the tens. In the problem you have above you have 7 in the millionths place and a 5 in the hundred thousands place. The previous poster us scientific notation to simplify this. 10^N is a number starting with 1 and ending with x number of zeros. So a million is 10^6 or 1 followed by 6 0s, 1,000,000.
Or to put it another way, 7 million 5 hundred thousand =7*10^6 + 5*10^5=7.5 million.
Or to put it another way, 7 million 5 hundred thousand =7*10^6 + 5*10^5=7.5 million.
