Scientific notation is a method used so we can show a very big/small number easier. Rather than putting so many zeroes, we put the number in the format of a number multiplied by 10^x.
Before putting a number into scientific notation, you need to know which number is a significant figures. In this question, there are two significant figure which is 39, all zeroes on the right is insignificant. We can move the decimal to convert the number into 3.9 * 10^5. Basically you need to move the decimal to the left if the exponent positive, but move it to the right if its negative. The exponent is +5 so we move the decimal points 5 places to the left.
