Well, let's first square 3.5x10^4 which is 1,225,000,000
Scientific notation is when you put a decimal place after the first digit and then say the answer is that multiplied by 10 to a power. This power is the number of digits that come after the first. In this case, the power should be 9 (if you count the 225 and all the 0's, you'll count 9 numbers).
Thus the answer is 1.225x10^9